Reshimming your valves depends on how much the valve is out of spec. There is a specific formula that you follow to determine the correct shim thickness based on the old thicknesses. As shims don't wear out, at least around here, you can swap them with a shop's supply for free.
As for determining shim thicknesses, you use a feeler gauge. There will be two feelers that fit; one a little loose and the other pretty tight. You want the looser of the two. Having a micrometre for measuring the shim thickness (and as a double check tool) is critical, and having one of those $1 telescoping wands with a magnet on the end is nice as the shims are magnetic. ORganization is critical to this and accuracy in measuring as well as labelling is critical. I'd suggest taping the original shims to a large piece of paper with a image of where you got it. Some motors have easily accessibly, cartridge-style cams that can be simply removed with a low possibility of disrupting the engine's timing during their removal&install. Other motors will torment you.
Not only is there a lot of measuring in reshimming a motor, the possibility of screwing up the motor's timing as well as dropping a bolt down into the cases requiring an expensive motor pull and dissassembly usually makes the average rider with decent mechanical skills take the bike to the shop. I hate to say it but experience is critical in this adjustment, and if you don't know what you're doing, don't bother.
