Lightly sand the surface and wipe it down with deglosser.
Prime the door if needed (check reasons in the post).
Paint the edges of the door first with a roller.
Paint the interior panels using a brush and a foam roller for a smooth finish.
Paint the vertical middle panel, then the horizontal panels, and outside vertical panels.
Let each thin layer dry before applying 2-3 more coats if necessary.
Remove tape carefully and touch up any imperfections with a small brush.
Reinstall the door and hardware if removed.