lash is just another word for clearance. adjusting valve lash is the same thing as adjusting valve clearance. You have to go in there with a feeler gauge and measure how much clearance you have, then you have to remove the cams and shims, measure what size shim was in there, calculate how much clearance was in there and then you can figure out what size shim you now need. dows that make sense?
MK3 TSRM On-Line
Check that out.