I did this recently, and it's not too bad.
Jack/hoist the Motor/Trans Assembly such that its applying very little force. It is here for safety, initially.
Loosen the center bolt of each motor + transmission mount, but not too much.
You can use the rear transmission mount as the visual guide; you want the bolt to be centered in that hole. In my experience, this one is key for clutch shudder reduction, and reduced engine vibe from harder motor mounts (implying it centered the other mounts pretty well at the same time.)
You then can carefully jack/hoist the Motor/Trans assembly, either lowering or raising it slightly to be more centered on the easily-viewed rear mount.
[I have heard of folks starting the engine to let it idle and self-center... can't speak for that method myself.]
After using your centering method, just tighten up the center bolts again, then release the jack/hoist.