Gradually, you should increase
the height of the box.
Modify this by lowering
the height of your box or step.
Choosing
the height of your box will depend on your current fitness level, but most women will want to start with a 14 - inch to 18 - inch box, while most men will want to start with a 20 - inch to 24 - inch box.
There are so many factors including your current strength, your height,
height of the box you're jumping from, your jumping skills, your mental attitude towards this slightly weird and possibly scary movement, your coordination and spatial relation skills to determine where that bar is in space in relation to you, etc., etc., etc..
Once you are comfortable with the height you can increase the difficulty, and improve your jump, by increasing
the height of the box.
Somehow
the height of the box gives athletes a reference or benchmark.
Varying
the height of the box adds variety and changes the focus from glutes (higher) to quads (lower).
height:
the height of the box in which the character is to be inserted.
Measure the flap against
the height of the box, and make sure they're of the same size.