The app is nice, provided you don't have a newer phone where the camera protrudes. That makes it a pain in the butt. I've leveled more machines than I care to admit between my house, my friends machines and the ones at leagues over the last few years.

I start with the front levelers most of the way down and the back ones about half way up. Then level side to side. Make sure when you're leveling side to side that if you turn the front right 4 turns that you turn the back right 4 turns. If you don't, you're going to end up with literally a twisted machine.

Once you get it side to side, then go front to back. If the pitch is lower than the 6.5 degree target, drop the front levelers first if possible as opposed to raising the rears. The goal is to get the machine as level as possible with the front levelers as close to being bottomed out as possible. A lot of people end up with machines that have the levelers too extended which makes for a wobbly machine.

I'd recommend a digital 6" level with a resolution of 0.1 degrees. I use a craftsman one I bought years ago, but you can't get them anymore. Something like this should be fine;

https://www.amazon.com/Husky-10-Digital-Level/dp/B015NE6R4A/ref=sr_1_16