Use a palm sander with fine grit to give the door a light sanding. Keep the palm sander moving. If you leave it too long in one spot, you'll damage the wood.
Safety note: Wear safety glasses when using a palm sander. A dust mask might be necessary as well.
After sanding, wipe with a tack cloth to remove all dust. Follow with a pre-conditioner that will help the wood absorb the stain more evenly.
After the stain dries, finish with a varnish to protect the wood.
Mount the finished door.