Solid lifters on a standard cam are easy, just tighten them down until you have resistance on your feeler gauge, about 7intake, 9exhaust would be my guess, you'd have to reference a manual.
If you have a new cam look at the cam card and see what it asks for, it will usually give a hot and cold setting. Start with the cold setting and then check after it has run and gotten hot.
Some people like to clearance with the motor running. I've found no problems with shutting it off, immediately removing the valve cover (remove the bolts while it is running, and then setting them with it off. You can use a cut up valve cover to keep it from making too big a mess if you do them running.
Hydraulic you'll have to reference the manual, it is usually something like 1/4 or 1/2 turn past zero clearance I think.
My personal advice? If you have hydraulic lifters (which most do), set them while the engine is warm. It make no difference to the lifter, but it is cozier than working on a block of ice in Ottawa. And make sure that you have each piston at TDC when you set the lifters for that cylinder. And ignore the advice about setting hydraulic lifters while the engine is running, even if you feel compelled to make a splash shield. The Opel hydraulic lifter just needs to be set with a small amount of internal spring compression, and is quite forgiving for at LEAST another full turn after that (if the lifter is in good shape). Finer adjustment than that shouldn't be required, unless you have a lifter that is worn in a particular setting position. And the advice about replacing the locking nuts is the best of all.