Installing the trim is way easier if I staple the shims into place before hand. In the picture, the shims don’t support the window (there are ones that do this but are mostly cut off), they just create a plane that will keep the sill and casings flat/vertical (or at least nearly so).
When I first started shimming the trim I just used a razor blade to cut the shims to the length I wanted. But this proved to be a pain because I not only needed to combine multiple shims together but they all had to be less than several inches long. By clamping a block to my miter saw fence 2.5” away from the blade I made 20 large shims into 100 or so 2.5” shims — with very little effort.
Just before I place the shims in the window opening, I grab a handful and arrange them from thin to thick. That way I can quickly grab several different thicknesses to create a combination that is the total thickness that I want.
Another gem I found along the journey…
PS. I would love to model this post after an infomercial— if only there was enough time in the day. At first, black and white. You see me attempting to cut a shim with a razor blade and making one of several embarrassing or dangerous cutting slips: nearly stabbing myself as I try to cut the shim on my leg; gouging the wall accidentally; Gintzuing off the bottom of the curtains. Then you would see a close-up of me cutting the shim and it falling to pieces (POOF and I’m left with a handful of toothpicks). Or maybe I successfully incised the shim but then it won’t break off— I’m left toiling, trying and trying to break the shim along my cut but to no avail; surrendering with my head shaking and hands raised in frustration. Then, POOF #2: from a cloud of glittery smoke my blog post appears in technicolor, light shining upon it, and you totally understand that how you used to use shims is like proto-man pushing a cart with square wheels! All of a sudden I want to work for a marketing firm or start a comic strip ;-)