Measure around the three sides of doorway(s), using the tape measure or ruler.
Add all the dimensions to calculate the linear footage needed.
Measure carefully and mark with pencil the various lengths needed on the molding.
Add 1/16 inch to measurements to account for saw blade width.
Cut the ends at a 45-degree angle, using a miter saw or miter box.
Sand the rough-cut angled ends of each piece.
Paint or sand the molding pieces.
Nail the pieces around the doorway, using a hammer
Fill nail holes with joint compound. Smooth with sandpaper and paint when dry.