you will need red scotch brite pad, 320 or 400 grit sandpaper,remove all trim,wash the door with soap and water, rinse and dry, using red scotch brite scuff pad,scuff the entire door untill the surface becomes all uniform where you can't see any shiny spots, wipe down with wax & grease remover or alcohol, blow it off,hang or lay it on a stand,blow off and using tack cloth remove any dust or lint right before applying basecoat(most basecoats have 1:1 mixing ratio)check the tech sheet or label on the can,I'd probably use fast reducer since it doesn't get too hot this time of the year,overlaping each pass about 50%, apply a medium coat ,allow it to flash for about 5-10 minutes come back with medium wet coat and let it flash about 10-15 minute depending on the outside temperature,usually black covers in two coats,if you still think the coverage is insufficient, apply one more medium coat and let it sit for about 15-20 minutes, mix the clearcoat( most cheap clears have 4:1 mixing ratio)if not, follow the tech sheet or label on the container ,try to use fast hardener since you're doing only one panel,run a tack cloth over the surface to pick up any loose lint or dust,overlaping 50% each pass apply clearcoat to the outside of the door,let it flash for about 10 minutes,shoot the jamb or the backside of the door shell and then the rest of the door,let it sit/hang for 5-10 hours depending on the environment, install the door on the car followed with the trim,don't forget to clean your spray gun right after you finish painting
btw, there is absolutely no need to apply any type of primer if there is nothing wrong with the paint on the used door
spray gun should have 1.3-1.5 fluid tip,
hope that helps,but if I were you ,I'd check the prices on the materials and supplies and getting few quotes before doing the painting yourself,cause you will have atleast $150-200 in materials ,and you can get the door shot with black paint for around $100-150 at most shops,if you just bring them a door shell without the trim
Stefan