File
Go slow, check often, file only one end, hold ring square to the filing surface, hold up to a lite with ends butted to see if you are staying square.
File from outside inward to keep from chipping a moly ring.
The second land ring, if cast, will file faster than the top, so don't over gap the fist one of them.
Get a ring square tool to go in your bore to check the end gap. Or use a cap off a pen to put the ring in the bore square to check end gap.
Deburr the ends every time you stick them in the block bore to minimize scratching the bore. Use a knife stone to deburr.
It pays to get the hand turn rotary wheel type tool and clamp it up in your vise.
You don't need the electric version and they are over priced.
Check the oil ring rails too, just to make sure they are the right ones as far as gap range listed with instructions. You should not have to alter them if they are correct size for your bore.
Check the oil ring expander to see it butts up with some tension. Depends on if it is low or standard tension how much, but it should have some.
Keep the rings organized by bore as the gap will vary .003" for every .001" bore difference.
Gap you rings before final assembly because it is a dirty process. Have some light oil on bores to catch dirt and limit scratching, then wipe out the bores with a paper towel.