Same spot, same time. Pick one place with soft even light and shoot there every time. Mornings before you eat are the easiest to repeat.
Same outfit. Something fitted you can actually see your shape in, like a sports bra with leggings or shorts.
Phone at hip height. Prop it on a shelf or a stack of books, then step back until your whole body fits in the frame with a little room above your head.
Use the timer. Three or ten seconds gives you time to get set and drop your shoulders.
Take all three angles. Front, side and back, with your arms relaxed at your sides.
Stand naturally. No sucking in and no posing. Keeping the same stance every time is what makes your comparison honest.