Take the door off its hinges. Place a flathead screwdriver against the rim of the top hinge pin. Tap the screwdriver up, using a hammer, until the pin is dislodged. Repeat on all of the hinges. Place the door on a pair of sawhorses or a large work table. This will make it easier for you to do your work. Have a friend help you with the door if it is especially large or heavy.
Remove the paint, lacquer or varnish on the door. This will help loosen the panel where the crack is located. Use some sandpaper, a paint scraper or paint stripper to remove the paint. You can repaint or restain the door later.
Scrape out any paint, filler or caulk that might be inside the crack. Use a small pocket knife to get inside the crack to remove all of the debris.
Screw one wood clamping block on each side of the crack. Use a screwdriver or drill to screw the blocks 1/2 inch above and below the crack. Center the blocks in the middle of the crack.
Apply a very thin coat of polyurethane glue inside the crack. Use a small paintbrush to ensure that you get the glue into the small ends of the crack.
Connect a clamp around the wood clamping blocks. Tighten the clamp until the crack completely closes. Allow the glue to dry and cure for several hours or overnight.
Loosen and remove the clamp from the blocks. Unscrew the clamping blocks from the door. Fill in the screw holes you made with some wood putty. Allow the wood putty to dry for several hours before you apply finish.
Hang the door back on its hinges once the finish is dry.