how do i make a save system?

I am not sure how to make a save system I am very new to construction 3 and would like to know how to save for instance how would you save a skin that you unlocked so you can use it whenever you feel

How did you make the skin or what is it? Do you mean in app purchases on Google store or are you talking about something you made in the game itself?

I think you should start with Local Storage.

It will save to your local storage when you unlock that item.

Then loads back when you start the game again.