How do I make a level select screen?

I'm making a level select screen and i'm confused on how to make one, i've got all the layers and buttons set up but i dont know how to lock levels that have not been unlocked. Can anyone give me an idea of how to do this?

See these tutorials:

https://www.scirra.com/tutorials/1160/p ... l-selector

https://www.scirra.com/tutorials/1234/level-selection

https://www.scirra.com/tutorials/311/level-select

https://www.scirra.com/tutorials/4791/level-selector

How I do selections and locking (in this case for spaceships) is below:

So "ShipSelected" is a global Variable as is "ShipsUnlocked"

The green icons are for showing what ship is selected

When ShipsUnlocked is greater than or equal to 1, then this ship can be selected, setting the ShipSelected variable to 1 so that this can later be passed into the main game layout and used to pick the sprite/name/whatever.

When ShipsUnlocked is less than 1, a sprite is displayed over the ship (in this case a padlock icon) meaning it cannot be selected.

This is pretty far from the cleanest or tidyest way to do this, but it is very easy to follow.