Cold water on a gentle cycle protects the open weave and keeps colours from fading. A mild, fragrance-light detergent is kinder to baby skin and to the fabric. You can skip fabric softener entirely — muslin softens naturally as it is washed and worn, and softener just coats the fibres and dulls the breathability.
For drying, tumble low or line-dry. Muslin dries quickly either way, and a gentle shake or smoothing by hand as it comes out settles most of the creasing. High heat is the main thing to avoid: it can shrink and stiffen cotton over time. Our pieces are pre-washed, so the bulk of any shrinkage has already happened before they reach you.
On stains, treat them sooner rather than later with a little cold water and gentle soap; the natural fibres release most baby messes without a fight. Wrinkles are normal for muslin and part of its relaxed character — no ironing required for everyday wear.