Before you start to cut the frame pieces, you will need to buy or cut wood strips that are about 1 1/2″ wide and 3/4″ thick. You can rip one or more 1×6 pine boards into the right sized strips, using a table saw. 1×6 boards are actually 3/4″ x 5 1/2″, so if you cut each board into four long strips, you will end up with three 1 1/2″ wide strips and one narrower one for cross bracing.
Cutting the frame strips
If you need to cut your own strips, use a table saw with a finish carbide blade. Use a miter saw or hand saw to get rid of any large twists, knots, or curves in the wood. You should try to get the straightest pieces possible.
Assembling the frame
Lay out the frame pieces. If possible, put the rough-cut edges facing into the winsert so that the outside will be smoother and the tape will adhere more easily.
If your window has a cross piece running across the middle of the frame, line up the cross piece of the winsert so that it it’s in the same position. (Measure the position of the window cross piece and then
mark the corresponding position on the side pieces of the winsert frame.
Using a 5/32″ drill bit, drill six screw holes, two at the top, in the middle, and at the bottom of each side piece. You can make a small plastic template the depth of the frame pieces (3/4″) with a hole to mark where the screws will go. Use this to make an indent in the wood before drilling screw holes. You may want to use a counter-sink so that it will be easier to make the screw heads flush with the frame, but the pine will be soft enough that this step is not essential.
Screw the frame together. You may want to use a bit of wood glue to hold the pieces together while you position them. Screw the top, middle, and bottom pieces to one side first, then put glue on the ends of those pieces and screw them to the remaining side. Use a hand screwdriver, if you wish, to tighten the screws so that they are flush with the frame.
NOTE: There are also other ways of assembling the frame (you could use corner biscuits, brads, etc.).
Check the squareness of the frame by measuring the diagonals to make sure they are the same
length; adjust as necessary.