Find "&"
Replace with "&tag=youraffid-20&"
It depends on how the original links are formatted though. If they are search result, go for find: "&keyword" and change it to "&tag=youraffid-20&keyword" so that you are not adding your aff link into the url in a bunch of times.
If they are individual product listings is depends on how you set them up.
A simple http://www.amazon.com/exec/obidos/asin/ASINHERE/ can be turned into http://www.amazon.com/exec/obidos/asin/ASINHERE/YOURAFFID-20/
If you want to post a sample of the url format you are using I can give you a better answer.