@CJW is completely right. You can do it however you wish.
Though in my game it’s a very complex system of variables, penalties, and dice rolls, with DCs(Difficulty Classes) being the determinant factor of whether or not they pass the roll.
Though that’s very complex, and trust me… it was a pain in my behind for days to code in without anything to go from. I had to work completely from scratch. But it works, and it’s pretty awesome in my opinion…
But like @CJW said, it’s your game, and do so as simple, or as complex as you see fit 
Here’s my complex and double-edged script if you want an example… it’s based off of Dungeons and Dragons… but I tailored it to fit my game
Hope this helps 
P.S: Thank you @Voldy for mentioning me
P.P.S: I just realized how incredibly confusing it was… so I made a simpler version of it 
Sorry…