Ensure the door is clean: To ensure the door is painted evenly and smooth it must be clean of dirt and debris.  A diluted dish soap mix works very well.  If you want to use a stronger chemical like acetone or mineral spirits it will be important to find the material used to build the door and decide if it is appropriate.

Prepare the door: If the door is already painted it might be appropriate to use a chemical stripper to take the previous finish off.  For wooden doors or doors with wooden components you can clean up dings and scratches with a fine grain sandpaper.  Use painters tape to protect the areas you do not want to finish, such as the hinges, glass, trim, or hardware.  If your door is fiberglass a bonding primer will be necessary to paint/stain.

Painting: Select a finish that is appropriate for the job by finding a product specifically for a door project, or for exterior projects.  Use a narrow, fine brush to apply stain/paint to the small areas requiring a bit more attention to detail. Use a wider brush for the panels, or larger sections of the door. Paint the sides, or stiles, with a smooth vertical motion.  Paint the top and bottom of the door, known as the rails, with a smooth side to side horizontal motion.  To blend the areas with areas already finished, gradually ease up on the pressure of the brush to feather the areas and blend them together.

Finishing: Allow the door and components to fully dry.  Remove the painters tape and touch up as necessary.  Determine whether a second coat of paint or a topcoat will be proper to finish to job.

Caution:  Ensure you are using a product specified for use with your door.  This will make sure you get a good bond with the finish and can avoid future bubbling, checking, blistering, or flaking.