For those of us who doesn't use Greenfox, here is how to give rep!
Video tutorial available here.
Source: Idea copied from Hallowsend & Pics made by me!
1) Leave a nice comment to the user (And ask him a question if you need help!
2) Click on the Reputation button!
3) You see the box that pops up, when you click the Reputation button. Leave a comment to the user.
4) After you have written a small comment, click on: "Add to reputation"!
5) You are done! The notification box shows you that you successfully have given reputation to the user. Remember to do that every time you see something helpful!