if you don't even understand the principle behind idle up flight, you're almost guarenteed to crash. Seriously.

If you have a simulator, I strongly suggest practicing on it ALOT before even attempting to invert your helicopter. Don't forget that once your helicopter is upside down, the controls all become double reversed (that is to say, one control direction will work in the logical method, while the other will be backwards. These switch around depending on what direction the helicopter is facing. If you think hovering nose in is difficult to master, hovering inverted is a nightmare! ).

To answer your question, though, most pilots will spool up the helicopter on the ground, put the throttle at mid stick (though this depends on how your pitch curve is setup) and then switch to idle up flight, and begin their flying routine. You can also do it in the air if your helicopter is setup properly. If it's not, you'll get a big jump (or drop) in altitude as soon as you switch modes.

Also, in idle up mode, the motor gets constant power, so if you DO crash, you're going in under power, so make sure you flick the idle up switch back, or you'll see smoke coming out of your ESC, motor, and even battery.