Ones like Landing Gear rub-ons, you just line them up carefully (maybe put some guide masking tape down the fork to assure it's aligned properly) then press them on.
Use a dry cloth and rub the backing paper hard before you remove it (but don't catch the edge!), ensuring they stick properly.
If they're bigger, you can use water with just a drop or two of detergent in to move them around, and squeegee all the water out from under the sticker (air bubbles too).
When I used to put them on the sides of vans and shop windows, I cleaned the surface first with acetone, but be careful if your frame is painted - it may not like acetone!
Just make sure there's no dirt or grit there.
For really long stickers, it sometimes helps to cut it in two (between two letters) and work from the middle out, doing one half first - that's maybe not a problem on a bike frame though.
