Measure the door opening
Choose the door type and material
Gather lumber, panel, hinge, latch, screws, glue, sandpaper, primer, and paint or finish
Cut the frame pieces to size
Cut the door panel or core to size
Assemble the frame
Insert the panel or core
Glue and clamp the joints
Reinforce the frame with screws or dowels
Let the glue cure
Sand all surfaces and edges
Rout or chisel hinge mortises
Drill holes for the handle and latch
Install the hinges
Install the latch and handle
Prime and paint or apply finish
Hang the door in the frame
Adjust the fit and hardware
Test opening and closing