Shims

Once all of the screws have been driven into the frame it's rather simple to remove the extra length of the shims. All you do is cut the shim with a razor knife as close to the vinyl window frame as you can without cutting it. Once you cut the shim you just snap it towards the cut and it will snap off level with the vinyl frame.

The same procedure is used for any shimming job. Doors, for example, are shimmed the same way.

One of the most important things, and I mentioned it in the previous post, is measuring everything to make sure you have it in square. Minor adjustments can be made with the screws and shims.

The easiest way to check for square is to measure from corner to corner. In the case of a window or door they are simple rectangles so the diagonal corner to corner measurements should be the same. If they are not the same, you no longer have a rectangle you have a parallelogram. Bad. Doors and windows don't look or work right if their frames are not square. Maybe most people would never notice...but if the side of a door I put in was rubbing the frame at the top and had more than a 1/4 inch gap at the bottom it would drive me nuts and I'd think about it every time I ever looked at the door.
So make sure everything's square.
You can use a carpenters square (a big metal L) to check as well...but measuring the diagonals corner to corner is easy.