It is very hard to do a diet at Subway, because they have so much bread and because of the discount for getting a footlong. And because their salads are so bad and overpriced. (Note that I consider that a footlong to be the equivalent to 4 slices of bread). But here is an attempt:
Breakfast: Either skip and just have a slice of bread with protein and cheese, or have a footlong egg and cheese and throw away 3/4ths of the bread (or save it for later). (Footlong with 1/4 bread is about 600 calories).
Lunch: order a 6-inch from the fresh-fit menu and eat only 1/2 the bread (Fresh fit, six inch with 1/2 bread is about 250 calories). Or if you had a small breakfast, have a footlong and throw away 3/4ths of the bread. No chips.
Snack: Eat some bread that you saved from before.
Dinner: Order a footlong and only eat 1/4 the bread. Don't order the tuna, because it has way too much mayo in it. Add chips and a diet soda.
After-dinner treat: more bread (sorry that's what you get if you choose this). Or maybe a cookie instead.
I would be willing to try it and see if it works.