Ok, bounties are started on questions:
Click on the "start button" beneath "add comment" which is placed under the question (not under the answers).
Then, chose the reason you are starting a bounty on the thread. If you want to award a bounty for one of the answers you have liked, choose the following option:
One or more of the answers is exemplary and worthy of an additional bounty
Then, you need to wait at least 24 hours to award the bounty to the answer you want.
However, @whuber's recommend (and I strongly agree) one wait until the grace period (8th day) to award the bounty; due to the reasons below cited.