The first step is to fold the headband strap so it loses some length, also accounting for extra length in the future. Pick somewhere on the left or right side so you dont feel the fold as much on your head

Remember that there are 3 different combinations to put the strap on your head, so you can start with the headband loose (bottom rungs) and then go up a rung as the headband expands again.

Personally I've used between 1.5cm-2cm folds.