velcro?! How old are these boots? Honestly, If you've had them for a long time, I would seriously consider purchasing new boots. It will make your riding experience way more enjoyable. You can find great boots are really decent prices especially this time of year. I always shop from Backcountry.com, check em out they're great people to work with.
But, what do these "shims" look like? The ones on my boots (2009 model) were, if I can remember correctly, about 4" long, and inch wide, and had an adhesive on one side to stick to the outside, of the inner boot.
If these are the same type you have, it should be relatively easy to get em setup once theyre in the right spot. Let me start with, these boots have an inner boot, correct? What brand/model are they? If ya dont have pics I can google it just to see what they look like.