I'm not a big tequila guy, so when my buddy pointed out he was making margaritas with Bacardi, I was on board. For the Ninja:
Bacardi to the first tick mark. I don't know what it reads, but it does wonders for my buzz.
Minute Maid frozen limeaid (the large one...bacardi makes one too)
"Splash" of Grand Marnier. Now, this is probably 2-3 shots for me, but my buddy's wife described it as a "splash" when describing the process, so...
Tap the top of the blender, add ice, and hold down until you hear no ice getting crunched.
One thing about the Ninja I've noticed: If you just hold it down with ice in there, the ice sometimes sits at the top. Pulse it to start, then hold it down. Pulse it a couple more times, then hold down to finish and it gets the churning action (might take a couple of times through that detailed process for the effect desired). I haven't figured out the physics involved, but I'm not fully satisfied the process is complete until the mixture is churning up and down.