Actually, the jam extensions can be your basic 1 x material. If you are doing Oak trim, you'll want to use Oak extensions.

What I do is once the window is installed, I measure all four corners from the face of the frame to the drywall. Now you have to take into account the entire plane of the drywall, not just the edge. This takes a little experience and is hard to explain, but a lot of times the edge of a piece of drywall may 'dip' down a bit. I'm talking about the last 2 inches. The reason for this is that if you just go by the actual edge, you will have trim that is cock-eyed and difficult to meet up with in the corners. So what you do is lean into the opening of the window, close one eye, and look up or down wall adjusting your head to where you can't see the whole wall, you should just see a point where the wall disappears. (like I said, this is difficult to explain in print). Then you 'project' that path to the tape measure. It is usually about 1/8" more than the actual edge. Now, this does not eliminate the possibility that the actual edge is what you should use, so watch for it. In fact, most of the time the edge is fine.

OK, now you have all four measurements. On the top you have a top-left and a top-right. Say the top-left measurement is 2 1/2" and the top-right is 2 1/4" (not unlikely). You will purchase a 1 x 4 piece of stock for this top. If all your windows will be the same roughly, and you're sure, for example, that none will be greater, you would just measure all the perimeter of the windows and add about 10% when you pick up your material. Sounds like you are only doing 1 window, but the info will be helpful for anyone else that will be doing more.

Now, measure your length from side to side. What you want to measure is the window frame, not the actual rough opening.

What I usually do is try to get a piece of stock that will wrap the entire window. If the window is too big for that, try to at least do it in two complete pieces (which you cut into a total of 4). What you'll do is start on one end of the board, cut the top piece, measure the side and give yourself a couple of inches extra, cut that, measure the bottom, cut that, and then the left side. So you have 'wrapped' the window with the same board and the grain will flow. It looks much nicer that way. (incidentally, I do the same with the trim. If you can't get a piece big enough, get one that is at least big enough to wrap a top and side.

Before you actually cut anything, select the best face of the stock. Then select the best edge. Now route a small radius where the best face and best edge meet, something like a 1/8" radius is nice, you can go smaller but don't go bigger. Route the entire board. If you have more than one board, it's a good idea to do them all at once, that way you're sure the settings are the same on the router. Finish the board(s) with whatever method you will be using for the trim (stain, laquer, etc...)

Now, cut the top piece off (measure twice, cut once :wink: )

The tricky part
Now hold the piece up to where it will be, the way it will be (the face you selected should face downward and the edge you routed should face you. Your top left measurement will be for the end to your left, and the top right measurement is for the end to your right. Measure from the front edge (routed edge) the distance, say it's 2 1/2". Place a little tick mark there, on the face side that you're using. Now do the same on the right. (if the measurements happen to be the same, you can just set a table saw to that measurement and rip it as is.)

Next, use a level or good straightedge to line up the two tick marks and use a utility knife to scribe a line from point to point. This will give you a tapered cut.

How do you cut it? I'll tell you my method, but it can be dangerous so be careful. I remove the fence on my table saw and freehand the cut. Make sure you are on the waste side of the line. The good face should be pointing upwards. I usually put the wood I'm keeping on the right of the blade, and the waste on the left, cause I'm right handed. Keep this cut straight, take your time. You're putting a scratch on the wood with a razor blade, and you have to split that scratch in half!

When you're done, hold the piece in place and verify that everything fits nicely and that

Now repeat the process for the bottom piece.

The way I like to get an exact measurement for the sides is thus: measure the frame on the right side, top to bottom. Now subtract the exact thickness of the top AND bottom pieces. Cut your right side piece to that length. Remember to use the right piece, so the grain flows.

Hold the right side piece in position (you can hold it outside the window, just make sure the proper face is facing the window and the proper edge is facing the room). The same top-right measurement will be for the top of this side. The bottom-right measurement is for the bottom of the side. Follow the same procedure for marking and cutting the side.

Do the same for the left side piece.

Putting it all together
Lay the boards on the floor just like they'll be in the window. The side pieces butt up to the top and bottom pieces. You will be drilling through the top and bottom pieces, into the ends of the side pieces. Hold the side piece up against the face of the top piece. Use a 1/8" drill bit and #10 screws about 1 1/4" long. Predrill your holes (the number of which depends on the width of the board; for 2 1/2" wide I'd probably just put two, for 4" wide I'd use 3). Now screw the pieces together being sure to hold them snug so there are no gaps. Don't over sink them either, or you'll split the end.

The pieces at each corner should match up exactly, width wise. If they don't, double check yourself. Assemble all the corners in this way.

When you're done, you'll have a top piece, the two sides below it, and the bottom piece. It will look like a box frame.

If you have an electric hand planer, it's a good idea to take the sharp edge off the entire outside of the box frame. This will help the trim lay down better if the frame sticks out a bit too much. You don't want to eat into too much finished wood though. I typically leave a 1/8" reveal when I case a window, so I only try to take off about 1/4" in from the edge, that leaves room for the trim to lay on a flat surface for nailing. You'll see what I mean later on, if you don't do it.

Now pick it up and place it in the window. Make sure it fits nicely. If it does, congratulate yourself. (I usually use something like, "Somebody sure knows what they're doing around here!")

My favorite method for attaching this new frame is a finish nail gun. Before you nail though, get yourself some flat, thin stock (I different materials of varying thickness) and you should shim between the rough framing and the jam extension. Use a 4' level if possible to make sure the side pieces are straight. Make sure the extension frame is up tight against the window and centered on the window. Start at a top corner about 3 inches below the top. If you need 1/2" of shim there, use your shim stock to build up to that 1/2" and put it in place. Nail through the shims. Use nails that are long enough to go through the jam extension, shim stock, and penetrate the rough framing at least 3/4".

On a normal size window, I'll do about 4 groups of these shims on a side and just 3 on the top. The bottom is usually resting on the rough framing. This shimming not only makes it easier to trim out, but keeps things from warping later on from the sun.

Almost done!
I know this seems like a lot of work, but it's not. It just takes me a lot of writing to describe it. I could show you everything in person much more quickly. After you shim everything out, get some scrap insulation and tear pieces of the 'fuzz' off. You don't want paper, just the insulation. Use a flat bar to pack it loosely into the gap between the rough framing and the extension. If the gap is 1/4" or less, just caulk it.

That should do it, you're ready to case it out!

Hope that helps.