Head over to - https://steemitwallet.com/~witnesses and vote for the witnesses that you want. Those listed are in the top 100, and if you know of someone that you would like to vote for that isn't in the top 100 you can add them by username in the box at the bottom.

Its very straight forward.

I had about a dozen votes that I didn't use so I voted for the bottom 100 that were still active with them. Not much of a strategy, but it gets the votes working and gives those at the bottom a better chance of getting a positive ROI.