My oldest sister was significantly over weight. She had her own gravitational pull; when she jumped up and down the song playing on the record player skipped, the song playing on the radio...Radio Free Europe (old joke).
My parents tried a bunch of different techniques all designed to minimize judgement and help her loose the weight with dignity...they hit all the bases: It is for your health, you'll feel better, we love you, we'll do the work with you, no snacks in the house, etc. None of it worked and, frankly, rarely does.
I can count on one hand the number of sodas I had in my house as a kid. I think the lack of sugar affected me
Then, 18 years or so ago, my daughter, then 4, innocently asks my wife, close enough for my sister to hear, "Why is Aunt Ann so fat?"
Everything changed and she lost a dramatic amount of weight which remains, I think, off.
So, I like all the suggestions so far but the magic solution does involve both an ice pick, honesty and you at some point: There isn't a template or checklist to follow and don't expect the first foray into honesty to be met with less snacks, probably the obverse.
If your daughter is willing, ask her what she would do.