For a NB you can use the inserts for 2 (wet not soiled) nappy changes because they don't wee too much and you're changing them so regularly -
1 change: the short insert with booster attached,
2nd change: the long insert folded and snapped on the shortest setting.
I'd put the wider part of the insert at the front of the nappy for a boy and at the back for a girl.
As wetting increases you can start using both inserts with or without the booster.
And as AandCsmum said they will shrink down a bit as you wash them... And they fit better when they have to curve around a bum!
HTH