If you are installing new patio doors, or weatherproofing previously installed doors, sealing them to keep out the drafts of a cold winter is a necessity. This step not insulates the inside of your home from cold weather, but in warm climates it also keeps cool air in. A basic procedure and little hand-eye coordination is all you need to get this project right. It won't take more than an hour to complete for each door you seal.
Measure the width of the outside of the door mouldings, using a tape measure. Cut a strip of still nosing, or another similar piece of moulding, wide enough to fit just under the threshold -- the space underneath the door -- to this length, using a handsaw.
Insert the moulding under the threshold and the two outside vertically aligned mouldings so it is exactly flush with the bottom of the doors. Glue the moulding in place, using a construction adhesive.
Apply a bead of door caulking, or exterior home caulking, to the outside edge of the moulding pieces. Run this 6 mm to 1.2 cm (1/4 to 1/2 inch) bead all the way along the edge of the seam between the siding and the moulding. Make a continuous bead by pausing to release the caulking gun trigger after it is fully depressed, and then continue on in your application after depressing the trigger again to restart the flow of caulk.
Repeat the caulking application on both sides of the moulding and across the top of the doors to form as continuous a seal as possible.