I've written before about how this thing was basically responsible for my daughter sleeping through the night pretty early on. The key is in its velcro sides, which snugly hold a baby's arms in at night so they're less likely to "startle" awake.
But TBH, my biggest tip (and one I'd wished I'd hadn't learned the hard way, lol) is to get at least two of these. Because inevitably, one night, your kid will have a blowout — and you'll want to be able to swap in a clean one ASAP so they can go back to sleep. In short? Always have a backup!
Get a Halo sleepsack from Amazon starting at $12.92. Available in multiple sizes and colors.