1. Go to your profile page.
2. Go to Edit -> Update your profile
3. Click on Main or About me, depending where you want to have your banner shown.
4. There is a small icon that looks like a tree. Click that.
5. A small pop-up appears. Insert/edit image
6. Copy the url of your uploaded (!) banner and paste it into Image url.
7. You can edit the size and such, if you want. Normally, banners have the perfect size so that you don't have to do anything about it.
8. When done, click the green button "Insert" below.
9. Click update! A notice in blue will tell you, if your changes were sucessful.
For a forum signature:
1. See above.
2. Same here.
3. Click Forum.
4. There is a text field. Write [img]here comes the link of your uploaded banner[/img]
5. Click Update! Again, a message will appear.
I hope this helps.
Edit: Here is a tutorial with screenshots made my the fabulous Hairann.
Hairann's tutorial^^