Field method: Deck of cards which are always easy to find on deployment. Place them under the flat bottom scope until you can’t fit more. Make sure all cards are centered under scope.
Tech method:
take 2 iPhones with a level app that show degrees and then level your rifle, switch to other iphone to confirm its same level ensuring by eye it’s perfectly in the same position as other iPhone. If phones are not giving same level data the app should have a calibrate feature. Calibrate them Identically this can be a step 1 also.
Once rifle rails are level place scope on rings and finger tighten to even ring gap spacing on tube clamp points. Now level with an iPhone. I usually depending on scope make take top elevation cap off so I’m on scope body not a cap. Depends on make.
Level and tighten, test and check test and check.
App i I use is theodolight Serves many purposes. Also map measure pro is an awesome app for long range shooter.