So you go in, you stand at the side of the counter furthest from the till, and you get asked which type of bread you want and whether you want a 6 inch or foot long sandwich.
Then you move along a little and they ask you what sort of filling (meat or similar) you want. They'll then usually ask if you also want cheese or not, and whether you want your sandwich toasting (warming up).
After that, you move along a little more, and you'll be asked what (if any) salad items you want. After that, you'll be asked what (if any) sauce you want.
As you reach the till you'll be asked if you want other things, so like cookies, drinks, etc.
My go-to subway of choice is a foot long Italian Herbs and Cheese bread, with chicken teriyaki and cheese toasted, with cucumber and red onion and lettuce, plus sweet onion sauce.