Here are some tips: https://www.thisoldhouse.com/painting/21016491/how-to-paint-doors-windows-and-walls
If you're repainting, you'll want to find out what kind of paint was used originally to determine if you need to use a primer. Here's a link with more information:
This should help you:
This may be helpful:
Outside doors or inside doors.?
While you certainly can protect the floor beneath and around the door and paint it in place, pros know that for smooth results you’d best invest the time to remove it from the frame. It’s easier to fill and sand cracks and imperfections prior to painting with the door on a flat work surface. And in this horizontal position, there’s not much chance of gravity producing unsightly drips and globs.
To take the door off, tap the hinge pins loose with a hammer and nail; the door should then slide off its hinges. Place the pins in a safe place for remounting the door after it’s painted and fully dry.
7. Remove the doorknob.
Painting a door—particularly an older wood one with trim and molding—generally requires some effort to prepare the surface. First, fill any small holes or cracks in the surface with a top-rated wood filler or spackle, a gypsum plaster and adhesive filling compound that shrinks very little after drying and can be painted without priming (and sometimes even without sanding). Place a small amount of spackle or wood filler on a putty knife, scrape it into the hole as evenly as possible, and let it dry for about two hours. Reassess if you need to add more spackle or filler and sand the filled-in spots smooth.
After any imperfections are filled, cured, and sanded, sand the entire door surface with 120-grit sandpaper, either manually or with a power sander. Wipe the door with a cloth dampened with mineral spirits and dry with another soft cloth to remove any surface dust. Apply primer if painting over oil-based paint so that you can switch to latex, if desired. When the primer coat is fully dry (preferably left overnight), gently sand the surface again, this time with a 220-grit sandpaper
Once the surface is sanded smooth, it’s time to paint. Rather than place the door directly on the work surface, elevate it by a few inches. This will increase ventilation and decrease any chance that a slightly damp paint coat will stick when you flip it over to paint the other wide. This is where Painter’s Pyramids come in handy. The sturdy plastic pyramids, strapped to sawhorses via convenient holes, let you raise the door by two inches and, when you flip the door to paint the other side, decrease the surface area in contact with fresh paint to tiny points.
Now that you’re ready to begin the actual work of painting, do the door all at once, without taking a break. If you pause for even a few minutes within a coat, paint can dry unevenly and result in a patchy finish. Latex paints can be dry and ready for a second coat after about four hours. Oil-based paints need 24 hours before a second coat can be applied. After the first coat is dry, pros often recommend sanding again, using 320-grit sandpaper. Remove any residual paint dust with a clean cloth and apply a second coat.
Hello these links might be helpful
How to Paint front door
How to Paint Exterior Doors
Hi Jan, hope these hep you out,
Here is a nice video tutorial for you: https://www.youtube.com/watch?v=h3g9FgeJNEc
Hi Jan. For best results, take the doors down. If painting while still standing, you could end up with drip marks and brush marks. Pull the trim up too. Clean every surface. Fill any holes and cracks with wood filler. Allow time to dry according to instructions on label. Then sand every surface, and wipe off all dust with a damp cloth. Next it's time to prime. Kilz would be a good choice for primer. Allow to dry completely. Lastly, it's time to paint with your color choice. Good luck Jan.
This will help for painting door trim: https://www.youtube.com/watch?v=uv2ftTnB5Bg