For the drowning one, I did that on one of my hit contracts, you have to push someone off a bridge or something. Most of the executions I never did manage, though. You have to be in certain positions, using certain weapons, to complete them. I'm sure if you look around you could find a list of how to do each execution. I made a wuick look on google, but nothing came up without me having to look for awhile, and I don't have time now or I would.