Add Extension Jambs and Trim Molding
Pick a style of molding that fits your home’s decor and create a frame around the window. Measure the length of each piece and use a miter saw to cut the trim to the appropriate length. Make your cuts on 45-degree angles so the trim will fit together with a nice picture-frame look. Attach the trim to the wall with finish nails. You may also need to add thin strips of wood trim (a jamb extension, see Step 1) inside the original window frame to cover the gap between the new window and the drywall.
At first, use only enough nails to hold the trim to the wall until you’ve successfully lined up all four sides of the frame. Once you’ve achieved the proper fit and you’re sure the trim will not require additional adjustments to fit, you can go back and add more finish nails, every 6 to 8 inches. Any slight gaps in the corners can be filled in with silicone before painting.