Open a beer with another beer. And not open both!
It is all about the trigger finger. This hand is on the beer that you want to open and is now called #1.
The beer you don't want to open (#2) will pop off the cap to your beer. Place smooth part of #2 cap under the cap of #1.
Do not do this. You will spill beer when #2 opens. Might work a few times but you will spill beer at some point.
Combine picture two and three. It is all about your trigger finger. Squeeze finger and slowly drop #2 acting as your lever. Your finger is acting as the fulcrum point.
Works best with an unopened full water bottle.
You get the idea. Tip: place #1 beer on a hard surface like a table first few times.
You have completed the process if it looks like this. Don't be lazy and get a clean glass. It taste better that way.