At a beginner level, simply taking the 2 fingers of your back hand (you only need to use 2 fingers with
sail sizes up to 5.0 m2) off the boom and letting the sail sheet out is the best way to depower your sail,.
In the same manner that you "ease the sheeting angle" in a dinghy or small boat, you simply allow th sail to sheet out.
Where you picked up the "over sheeting" to depower is for much larger sails, where if you sheet out suddently it removes most of the mast foot pressure from the board and can cause major crashes/issues.
When planing into a jibe at very high speed, on much larger sails, oversheeting can be the thing to do.
It depowers the sail some, it pulls the board into the carve turn nicely and sets the sailor up to enter the jibe with lots of speed and a constant pressure.
For now, simply ease your sheeting angle with your back hand.