piston rings

Is there any guide to positioning the gaps of the piston rings in certain degree against eachother

120 degrees or so?

and how to mount piston rings correctley on the piston?

You do realise there was a post IMMEDIATELY below your posts, which would have meant it was the TOP, asking EXACTLY this question?

Other than checking the oil rings' expanders are butting and not overlapping, and the gaps are within tolerances, I don't worry about it for the same reason I gave immediately below. I do know others may have "secret" installation angles they swear by, but...

Rings can be installed by hand in almost every instance*, but if you aren't practiced and/or have soft fingers, proper ring expanders are cheap to buy. From the question, I would suggest the machine shop both gap the rings, if required, and fit them to the pistons - should only be a few dollars and cheaper than the tools that may be required.

If you wish to fit them yourself, videos are worth a thousand words, and there are many guides on-line. It's a lot easier if you hold the connecting rod, with piston attached, in a soft jawed vice, and work from the bottom up.

*Cast ron rings are brittle as **** and some aftermarket ring suppliers recommend they be used, to avoid over-stressing the rings