I am so very, very sorry to hear this. I just went through this, so if you want to ask anything, let me know.

Here's a good page on what to do, when:

http://www.messybeast.com/euth.htm

If you scroll down to "What to do, when", it gives you body language signs to look for to see if your cat is in pain or ready to go.

Here's a thing about how cats feel pain, and some of the things they do:

http://www.messybeast.com/pain-perception.htm