I will skip the exact steps on how this works, because I feel like a much better tutorial is available at a the psi wiki. I would hate to plagiarize what they have.
However, the software you will need to download for this to work is available here. If you wish, you may start by downloading these files
Googletalk
PSI
The pros: You can use just one program to have all of your IM programs open.
The cons: This program does not support some advanced features, such as video chat.