Gratitude
The following tutorial on using IRC was created by yasminm. Please direct your thanks in her direction, just as we are. From the mods of Yaoi Daily, thank you, Yasmin. Please add her IRC Tutorial LiveJournal entry to your memories for future reference, if you wish.
The Dummies Guide to Leeching Manga From IRC by Yasmin M.
Introduction
So you're probably saying to yourself: "Gee, I'd really love to get at all the lovely yaoi/BL crack, but they're all distributed on IRC! I don't do IRC! It looks so complicated... isn't it?"
First of all, you need an IRC program. I use mIRC, which is pretty newbie-friendly. There are IRC tutorials available, so I won't go into the basics. I write this guide with the assumption that you're at least able to install mIRC and connect to a network. But worry not! I'm not so unkind as to leave you floundering in the waters. :D
Okay. So you want to download manga. But which one? It's best to make up your mind before you get on IRC. A good place to start is tokudane, which has a list of scanlation groups and where to find them.
For the purpose of this exercise, I'm choosing to download chapter 2 of Rin! The scanlation group is Mochi Mochi, whose channel is #kodocha at irc.irchighway.net.
TIP: Often, you don't actually have to go to a group's specific channel in order to find their releases, especially among BL fans. It's very likely that you can find a particular BL manga in another channel. The larger and more established a scanlation group is, the more likely you'll be able to find bots/people who carry releases from other groups.
That's why I'm going to #nakama instead, which is also on irc.irchighway.net.
DISCLAIMER: I am not expert, nor am I a techie. I just know what works.
Let's Go
First, open mIRC and run it. This window will pop up:

Just click on the X, unless you really really want to register.
Now comes the nitty-gritty. You'll see something like this:

Now, I already have the right network address. But when you first use mIRC, that won't be the case. You can try to find it among among the list of networks, but here's what I do:
Click on "Add" (the one next to the "IRC Network" drop-down menu), and this window will appear:

Copy-and-paste "irc.irchighway.net" (or whichever network address you want) onto the text area for "IRC server":

You're perfectly within your right to fill in the description as "YAY CRACK" or whatever, but I think this'll make life easier. Don't worry about the rest. Click on "Add", and you're back:

Fill in your name, e-mail address, nick. Again, don't worry about the rest. Click on "Connect to IRC server", and something like this will appear:

Eventually, once you're connected, this window will pop up:

Just type "nakama" (or whichever channel you want to go to) and click on "Join". Or you can do it the hard way, like so:

And you're in #nakama! It's usually a good idea to find out the rules first:

Or you can just skip it and type "!list", which gives you a list of all the active bots. Think of them as teller machines that dispenses manga, but with real people (who are generous and should be appreciated properly) at the other end.

Hit Enter, and lo, you get a whole swag of bots to choose from:

Unfortunately, I've found that just because it's listed, it doesn't mean that I can actually connect to a bot. Sometimes they're behind firewalls (see "Tannosaur" above).
In any case, let's start from the top of the list:

To connect, just highlight the "trigger" with your mouse. The text will automatically be copied. Make sure you get the entire thing, including the /. Please note that sometimes bots have more than one triggers, so beware of accidentally copying the other triggers.
Once that's done, paste (I use ctrl+v, but you can also use right-mouse click) the trigger just so, at the text area at the bottom of the window:

Hit Enter. If all goes well, this window should pop up:

Click on "Accept", and a chat window will open up:

PAY ATTENTION TO THE INFORMATION GIVEN. I can't leech off this bot, because the person has set the minimum CPS to 3500KB. I'm on dial-up, yo.
So it's time to try again. CLOSE THE CHAT WINDOW, then go back to the bot listing in the main #nakama window. I generally work my way down the list, or find a bot that's worked for me in the past:

Success! I can deal with this minimum CPS limit:

Now comes the tricky part. I recommend typing "help" and hitting Enter -- a list of bot commands will usually appear. This is my down-and-dirty guide, so I'm going to get straight to what you really need to know.
First, you need to find out what manga is available from this bot. Type "dir" and hit Enter:

Sometimes, a list of available files immediately appears -- especially if the bot carries only recent releases. More often than not, however, you get a list of folders/sub-directories:

Because the Rin! chapter has been released a while back, logically it'll be in "YAOI MANGA" instead of "NEW YAOI". So highlight the text and paste it, adding "cd" in the beginning, and hit Enter:

(To help you remember this command, think of "cd" as a shorthand for "see directory".)
Okay, so now you're in "YAOI MANGA". To view a list of what's actually in there, type "dir" again and hit Enter:

This person has organised her manga files according to scanlation groups. I already know that Rin! is being scanlated by Mochi Mochi, so I highlight the folder/directory name...

... and paste it onto the text box with "cd", hence: "cd MOCHI MOCHI". And then you go through the entire rigamole of directory listing again:

Finally! A list of available files! Copy the name of the file you want to download:

Paste it onto the text box, with the command "get" (commands always go in front):

(Pay attention to how long you have before the chat (DCC) automatically disconnects.)
Hit Enter. Assuming that the queues aren't full, you should be added to the queue:

The chapter scanlation has been broken up into two zipped files, so I'm also getting the second half. You're generally allowed to queue up to two or three files at a time:

This is important. Once you're done getting into queue, type "exit", hit Enter, and close the window:

Or just click on the X:

Don't worry about losing your place in the queue, because the bot will remember you. DO NOT LEAVE THE MAIN CHANNEL, which in this case is #nakama.
Why the closing of the DCC chat window? Because the people who're maintaining the bots aren't very happy about chat windows cluttering their main program window, especially when they can only allow so many users per bot. If you read through the main bot listing, you'll notice that some have messages to the effect that "campers will be banned!" Well, this is it.
Now, all you have to do is wait for the file to start sending, which can take a while. As a security measure, the default setting for file-sending in mIRC is such that you have to approve each and every request to send you a file. If you're paranoid about catching viruses and malicious thingamies, you can park yourself in front of the computer for up to an hour, waiting for mIRC to notify you of a "send file" request.
Me, I think it's more practical to change the DCC settings. This is how you do it:
Click on "DCC".

Click on "Options".

And this window will pop up:

The default setting for "On Send request" is usually "Show get dialogue". I've already changed mine to "Auto-get file", and made sure that "If file exists" is set to "resume". I've kept the settings for "On chat request" as it is, because I really would hate to be accosted by some weirdo.
Eventually, hopefully, the file you desire will start sending:

Of course, sometimes life doesn't work that smoothly. Let's say that you want to find chapter 4 of volume 8 of Love Mode. And so you go searching around the appropriate sub-folder/directory of a bot, only to find that it doesn't have what you want:

Damn it, life sucks! But then you remember that this person also has the scanlations for Gravitation Remix:

Gimme the boxcutter sodomy, baby. Or, hell, maybe you just went into the wrong sub-directory and want to backtrack. But wait! How do you go up one directory? Well, this is how:

Type "cd ..", then hit Enter. And you're back in the main directory:

Huzzah! Happy searching. Just remember that these DCC chats tend to close automatically after a minute or so.
As I've already mentioned, sometimes you'll run across bot messages warning you that they're behind a firewall:

You could copy "/dccserver +sc on 59", paste it onto the main chat window, hit Enter, etc:

Personally, unless I'm truly desperate, I prefer not to go through that much trouble. I'm lazy that way.
DIRE WARNING: Make sure you copy-and-paste the triggers properly. Or you'll end up like "kitsuke" here:

mIRC automatically saves all files sent to you in the "download" folder:
Program Files > mIRC > download
You can find the files you downloaded there when you're done, or change the settings so all the files will be saved to the "Ghei Crack" folder or something.
If you can't get the entire file off someone for whatever reason, don't fret. Just find someone else who carries the same file, under the same filename -- then resume your download. You can do this via the same way you would download any other file. mIRC will automatically resume from where you left off if you've set it up properly (the "If file exists" thing, remember?).
And I'm done! Questions? Suggestions? Offers of paracetamol tablets?