Here's how I do mine. Others might have a better technique.
First wipe down rings and scope tube with rubbing alcohol to remove any grease.
Hang a plumb bob 15yrds or so away depending on your scope's minimum focus distance.
Setup the rifle up on bench vise or on a bipod or something stable so it doesn't flop around.
Do a rough setup of your scope rings and see where they need to be on your base for natural eye relief.
Lightly snug down the scope rings to the base, just finger snug should be fine for now.
Sit your scope back on the rings and see if sits with no signs of daylight between the rings and scope tube. You DON'T want to torque down your scope rings on your nice scope if they are mis-aligned or you can wreck the scope tube.
Once that's good, loosely attach the top of the scope rings, enough that you can rotate the scope but not so loose that it will slide around.
Setup behind your rifle so that it feels comfortable while sighting down the scope. Once you have your position, gently rotate the scope until the elevation line of your reticle is perfectly aligned to that of the plumb bob.
Snug down your scope rings and attach the bubble level just loose enough that you can rotate it with just a little effort and it will keep its position without slipping.
Now setup the rifle so that the reticle is still square with the plumb bob. Without disturbing the rifle's position, level the bubble level and snug it down.
Verify that the positions haven't moved. If all is good, torque down the scope rings, alternating to the opposing screw, and also the bubble level to approx 20 inch pounds. Verify once more that torquing down the screws did not mess up your settings.
You're good to go now. Just get into the habit of checking for level before every shot.
Don't rely on your scope caps/turret to be perfectly square to your reticle!