the way i found out (or bengan told me) works as follows:
you get the kitee on its back to the giant, non moving U.
then you pull in one line (frontline works better), about an arms length
and bring one tip closer to you.
the kite starts moving a bit. from now on, a bit feeling is required - you work on the line in your hand and on the backline of the other side - make the upper tip catch some air, it takes a few tries.
as soon as the kite turns sideways and stands up, just sheet in and launch it - no need to wait until it drifts to the edge.
I have an Aero2 20 - And when I can get it on it's back, I feel like I am the luckiest man alive! If you can do that - You are 75% of the way there.
As for "no need to wait until it drifts to the edge" - If you can actually get it to launch earlier than the edge of the window, just be careful of how far away from the edge it is before it takes off. If the winds are at the upper end of the kite's range, you may be going for a ride.
Otherwise, it is just pretty standard description of water relaunch for LEI kites.
If anyone has a trick for getting the big Aero2s to lay on their back, I would totally appreciate it if you would share. Man, you have to be FAST.
My best advice is to learn to fly a smaller kite well before moving to an LEI above 15m. The less you have to relaunch a bigger kite, the happier you'll be.