for the most part, i use a wash out blue marker. white with blue cap. Dritz maybe. still, test on all fabrics.
Micron pigma pen .05 in size to write label. heat set few seconds with iron too. i always do. works well. i was in swaps for years and no problems.
also i use a Hera plastic marker for lines like cross hatching. it makes an indention in fabric, nothing to wash out.