Although i'm not converse with this particular engine, measuring for the correct shims is in theory the same for all shimmed engines. Some have shims on top, others have them below a follower, they are generally referred top as bucket and shims.
Take a measurement with good feeler gauges, not some tatty old bits of rust that have been hiding in the bottom of the tool box for six years.
This is done as mentioned earlier in the post, with the piston at T.D.C. [top dead centre, this is when the piston is at the highest point of it's travel] and the camshaft lobe is in an uppermost position in relation to the piston, which means that the particular valve is fully shut.
Take the measurement with a feeler gauge that slides into the gap with a good fit, you shouldn't be forcing a feeler into place but it shouldn't be sloppy. Think of a perfect hand in glove fit. You may need to use one or more feelers together to get the right measurement.
Write the measurement down and note the cylinder/valve. Remove the shim and measure it with a micrometer. Add the sum of the shim and the clearance taken with the feeler gauge. From that sum, subtract the specified clearance and that will give you the size of shim required to re-fit.
In some circumstances an exact shim is not available, these can be corrected but not in general for the home mechanic so i won't go into that right now. If the exact shim is not available, go for the one that leaves a slightly oversize clearance. A shim that is too tight will not allow the valve to close 100%, this allows gases to pass and can burn out a valve, lower compression pressure and basically be incorrect.
The first engine i shimmed was a Honda CBX1000 bike many years back, 24 valves and if i remember rightly, about 18 of them were rattling, oh the fun of it and it was a favour for a mate too.