You have a relatively flat door? It will be easy to paint.

For uniform results, it is to prepare the paint.

Start by protecting the floor, walls and furniture that could be achieved by painting.  

Use newsprint or plastic for protection.  

Remove paint residue with sandpaper (choose a paper preferably medium grain). Discard remaining dust with a soft cloth. The paint will be applied from top to bottom.

Start with the top corners and then move towards the bottom.

Finish the sides, spreading the paint carefully to avoid overloading or sag. Let dry and then proceed to a second layer if necessary.