I don’t think it’s explicitly explained in the original books, and every film/tv adaptation did them differently too. I haven’t read any of the Brian/KJA books and maybe they explain how, but most people don’t consider them canon.

My understanding of it when I read the books was the spice gives them similar prescience abilities to Paul. Effectively allowing them to see what's ahead and navigate before it's an issue. Especially because they're flying faster than light so they can't rely on their eyes as they'd crash into something by the time they actually see it.

I have a hazy memory of the book explaining they were once humans addicted to spice, so presumably they have some controls.

They have controls within the spice tanks to utilize as described in books late series frank Herbert line and many times in Brian herberts line

Computers were banned so it’s hard to say. Frank Herbert really doesn’t go into it as that wasn’t important to the point he was making.

I don't think it's ever really explained. because if these guys can fold space like that, you shouldn't even really need spaceships. that is, if they're creating some sort of finite and bubble around them that travels through the fold, as opposed to bringing things through that they're touching or something. but I still don't understand why those ships would ever need to leave atmosphere