I've never found one pop 900's or multiple flat spins to be reliable. I hate tricks where you yank a line and pray it does what you want.
For reliable multiple flat spins I learned like this.
Get a pitch stable kite.
Go out in light winds and practice pinwheels.
Practice trying to tend the pinwheel on the way down and keep it in control.
Add more and more control to your pinwheel until you are in total control and its not really a pinwheel anymore. It's multiple flat spins.
The key to multiple flat spins is to keep the kite moving down wind the whole time.
To keep the kite moving down wind the whole time while remaining in control, you need to practice down wind glides over and over and over. Multiple flat spins are a down wind glide where the kite is spinning.
This video shows what I'm talking about.