To fix a small, thin crack, you will need:
- Filler - you can get special filler for small cracks.
- A thin tool like a thin palette knife or a wallpaper scraper.
- Sandpaper - probably some that is fine and some that is less fine.
- Newspaper to protect the floor.
- The paint you kept from when you painted the wall. Otherwise, good luck matching it :)
- Lay out the newspaper to protect the floor. Protect whatever you need to protect!
- Use the thin tool to open up the crack slightly. You want to make it big enough that you can totally fill the crack, and you want to be sure that there is no debris in the crack. It probably needs to be a couple of millimetres thick, but you'll get idea.
- Fill the crack with the filler. You want to totally fill the crack and have a little over for luck.
- Wait for the filler to dry.
- Sand the wall so that it is flat. Use fine sandpaper at the end of the sanding for a good finish.
- Paint the wall.
- Pretend nothing ever happened.