I am sending this to you again, completed (there were a few things I had forgotten, so i am doing it as I run it). You may not be able to do it on your own right away, but this way once you understand everything and are used to it you can when I'm not there.

It looks long and complicated, to you. But the idea is, since you already know some of the steps, you look here for the rest. OR you just decide not to go online (IM) unoless I'm here >.> :p.

download.com
Search "trillian" enter
"Trillian 3.1;" the second option, scroll to the right and click download.
"Run"
[program downloads]
"Accept"
Change directory (text input box) to "C:\my documents" . Never use periods or quotation marks when somebody tell you to type or copy something unless specified.
"Go"
"Yes"
[program begins to install]
"No"
"Yes"
[program finishes installing - quick]
"Launch"
Click on the box in the taskbar that goes to Trillian. Boxes are always labeled. Anytime you need to, click the box to get back to that.
"Trillian Basic"
click any random picture and click next
Uncheck every box except the bottom one, which says "Yahoo! Messenger."
Click next
Enter your screenname and password. ENTER THE SCREENNAME ONLY, NOT THE FULL E-MAIL ADDRESS. NEVER ENTER ~"@BLAHBLAHBLAH.COM"~ WHEN SOMETHING ASKS FOR YOUR SCREENNAME (except in MSN messenger, which you don't use so ignore this exception.)
Click "next." It is on the BOTTOM of the window not below the text input boxes. If you need to, keep clicking "next" or "okay" until the blue dialog box goes away.
Click "okay" on the windows security alert.
Minimize your windows. You do that by clicking the __ next to the x where you would close it (actually, it goes "__," "[ ]," "x." The minimize/line is the leftmost button.
Go up to the top to the "file menu," and click "Trillian," then "Trillian Prefrences."
Click the icon that says "Contact List."
At the bottom of the window below the line, reverse the check marks in those three boxes. That is, uncheck the first two bows and put a check mark in the last box which says "Show Contact List as a task in Windows taskbar when not docked."
Click close.
Double-click on the screenname of the person which you wish to IM. This will be in the contact list, which will show up when you click the box at the bottom of the screen in what is called the taskbar if it is not showing already. The same thing applies if you want to open again the windows which you have minimized in the previous step. Another window will pop up. Start typing.
If the person is online (on Yahoo!), the circle next to their screenname will be red. If they are not online, the circle will be lighter; transluscent, and under the heading of "Offline Contacts." If a person is not online, you can IM them anyway, if you wish, on Yahoo! (only). They will recieve the message when they sign in, in a pop-up box just like the one you got when I added you to my contact list.*
< If you log on to the SAME computer again after it logs off, you will be able to get back on to Trillian just by downloading it. You will not have to enter screennames and passwords or change options. As a matter of fact, it may stay logged in. This does NOT apply if the computer has shut down.

*If they sign in from web messenger (not Trill), they will not be able to see the offline message, but they will know you have sent them SOMETHING. If they can/have downloaded something they will see it. It is most likely that you should ignore this "footnote."

To add another contact
In the Trillian contact window, which has the icon of to yellow circle-type thingies on the taskbar, click the menu option "Trillian," then "Add contact or Group... ."
Click "next."
Make sure the scroll-down box says "YAHOO," if applicable (I can't see how it would be that way since I have all four IMs), and highlight (click on) "Root."
Click "next."
Enter the person's screenname, and in the box below that, you can, if you wish, send them a message telling them who you are and/or why you have added them to your contact list. If someone adds you (you will know by reading the message), you can IM them in the same box that pops up first before deciding whether to let them add you or not.
After you enter their screenname and the optional message, click "Done." They will appear on your buddy list until they deny your request if thta is the case. Otherwise they will be there until you delete them (right-click and choose "delete.")