IIRC, it's pretty basic if you have the shimming tool, like the valve shimming plate (I have one, but it's not mine, it's Calum's).
You attach the plate, then use a sort of dial caliper gauge and measure the difference between the grooved shim (they're all the same size), and the top of the valve itself on the other valve. Then you know what size shim you need. I measured 3 times each to triple check the numbers and ensure I bought the right size shims, or could reuse the shims I took out.