On whatever the action is that completes the level system > go to layout > layout name
Do this in each level for the next level. Same idea for the level skip, except that whatever it is that allows the level skip is where you put in the go to layout.
so say they reached a door that leads to next level setup the condition on collision with object door and action system go to layout level 2
if they got a certain item in the level that allows the skipping of level 2 add a sub condition with a variable setup for that item being obtained
so conditions:
on collision with object door
if level skip is true
Action:
system go to layout level 3
does that make sense?