[SCRIPTERS] Character "fly" system like gmod or minecraft.

Hello scripters i need some help: Im trying to make a game like gmod here is a link to my other post HERE if you want to know more about what i want. So in my game is there a way that you can make the player fly and stop flying onfuntion button press or double jump? because i want to be able to make my player have control on his/her y position as well as normal controls on a function that activates FLY MODE. In other words something like minecraft fly system or gmod fly system please help....Thankyou