I agree with Rick above. The technique is simple: sand the surfaces and the apply more coats.
The better your sanding, the better your results. If you are inclined, start with rough sandpaper and then move to smoother sandpaper (say 320 grit). For table tops, I use 400 grit for the last grit of sandpaper. It should be smooth to the touch with no bumps. It should feel nice to the touch before you put more finish on. Some folks like to finish with fine steel wool (0000). Any blemish you see before applying more finish will be magnified by the sheen of the finish.
Assuming the finish you purchased isn't for a spray gun, then take note of the solvent suggested. If the finish leaves thick ropey trails on a test piece, mix some finish and a little of solvent together (in a separate container) and try again. If you have too much solvent, then the finish will be smooth but flat. Somewhere between ropy thick and too much solvent is a sweet spot for applying the finish. (Note: the sweet spot's mixture will vary depending on temperature and humidity so what worked one day may be different the next.).
Beside whatever it says on the side of the can of finish, there may be a couple of gotchas.
One, if there is gunk on the door or it isn't clean, your finish may not stick everywhere correctly.
Two, especially on the edges, it is easy to sand through the dark finish into raw wood. That is tough not to do and sometimes can't be fixed other than to use a bit of opaque color to hide that.
If you are horrified by the first pass, you can strip the door. I suggest trying a few sandings and coats first (and if still horrified, then strip it).
It might be worthwhile to remove the door and work on it horizontal. The better the lighting, the better your seeing, and the better your seeing the better your work. Use raking light to see the condition of your work every chance you can.
If you wish to dig a bit deeper, I suggest looking at Bob Flexner's books. If you can't decide on one, take a look at 'Understanding Wood Finishing'. Best of luck!