The first step is to fill in the gaps at the tops and bottom, inevitably there will be some imperfections in your wall so the gaps are normal.
Use a small bead of caulk to fill in the gaps and a credit card to smooth the bead and push it into place.
The next step, the nail holes. Use your fingers and some spackle to fill in all of the little indentations and nail heads.
If there are any nails sticking out, use a nail set to gently tap it below the surface.
Once the holes are filled and the spackle is dry sand any blobs or irregularities.
When you're finished, all you have to do is touch it up with a little paint. I'm Mrs. Fixit and it's just that simple.