First of all you need to decide what bounty type do you want to participate in. Signature campaigns mostly require rank of Jr. Member and higher. Twitter campaigns require 500+ followers.

Next. You should find some reliable project with a bounty campaign. Go to some ICO tracker and choose the project with a bounty campaign. I recommend to use icodrops.com. There you can find links to bounty threads. If you want to find the thread by yourself just google " bounty bitcointalk".

The last but not least thing is to register to the particular campaign. The instruction how to do that you will find in the bounty thread.