The first thing that you want to do is to clean the door. Use a solution of water and vinegar or ammonia and clean the door with the help of a spongy scrub. Get a new one so that it works well and does not leave any dirt that may be in it previously.
After you have thoroughly cleaned it, it is time to wash it with water. Do not use too much water but make sure that you get rid of all the dirt on it. Let it dry completely afterwards. Also if it is rainy season, it best that the door is refinished later as the moisture from the atmosphere can damage the door and your washing of it will aid that process.
The next thing to do is to sand it down to make sure that it has a smooth look to it. You can use the standard sander that is used on wood and can later use finer sandpaper in order to give it a clean finish. Wipe off any debris that may be there.
Make sure that you cover hinges and the door knobs with some paper tape so that the refinishing process does not damage them in any way.
Now look for any kind of cracking in the door. If there is any, fill it in with some wood putty. Make sure that you sand off the excess when the door is dry after the filling.
Put on Primer
Afterwards, put on a primer. You can use a small paint brush or a small roller that is used for painting purposes. Both should work well.
After you have allowed the primer to dry and have sanded the door lightly, give the door two coats of paint in the desired colour. Make sure that you allow the first one to dry well before you apply the second.