Pull up any carpeting or the current flooring from around the door jamb area. Use pliers to pull up carpeting or a pry bar to remove old wood or vinyl.
Set a piece of scrap wood from the new flooring, as well as any subfloor material next to the door jamb. Mark the door jamb with a pencil at the height of the new subflooring and hardwood.
Cut the door jamb with an undercut saw to remove the wood from the door jamb. Carefully pry out the wood.
Slide the new flooring and subfloor underneath the door jamb.
Measure the flooring for the doorway, including the space under the door jamb. Subtract 1/8 inch from the flooring size to allow for expansion of the wood during heat.