I agree with all the advise given so far and would like to add one tip.
Painting doors can take a bear of time because you have to wait for one side to completely dry before you can flip it over to paint the other. If you do it too soon you'll ruin the first side. I have a trick for painting both sides almost simultaneously without ruining either side.
Lay the door flat using two sawhorses, but do not lay the door on top of the horses. Instead screw two bolts half way into the bottom of the door, driving the bolts up into the door, not thru the face of the door. Drive a 5-inch lag in about 3 inches, each a few inches from the both ends. Do the same on the top end but use one lag in the middle.
Now lay the door flat on the horses but support the door with the bolt ends. The door will not flip over and will be held up with the 3 bolts. Paint one side and when it's just starting to cure (within about 15 or 20 minutes) grab the two bolts and flip the door using the top bolt as a pivot point. You can also do the hinge side and the striker side of the door with no problems. The whole door will dry at the same time and more than cut in half the time needed to paint a door.
When dry, remove the bolts and rehang the door. You will not see the holes left by the lags since they are on the face edges of the bottom and top.
I worked thru college doing painting and I needed to get in and out in one day. This was one of many tricks to get done quickly and efficiently.
Good Luck