You Tube has many tutorials on how to fit various nappy types.
Start with the nappy sitting just under the child’s bottom, not pulled right up at the back.
Pull the front of the nappy up, reaching past the belly button.
Pull the side tabs up and across before snapping or velcroing in place. The nappy should be firm but not too tight. Two fingers should fit at the waist, between the stomach and the nappy, it is ok for a small gap when the child stands/moves.
The leg elastics should sit in the leg creases, between the top of the thighs and the bottom. No gaps should be visible in the leg creases, if there are, the bottom snaps need to be tightened or the length of the nappy needs to be adjusted.