Once you reach high enough Steem Power every time you upvote a post a "voting power slide bar" will appear and you can delegate how much of your voting power you can give to a post..
If you want to just check how much voting power you currently have this is a useful tool...
www.steemforthe.win
then just type your name
Thank you for sharing this information @mithrilweed I love the fact that you used your upvote for @bdl1165 to show how the slider works. Great work my friend :)