https://www.dharmatrading.com/.....linen.html
You need a dye for cellulose type fiber. It is not hard, and if you don't have experience with dyeing, I would go with the iDye recommended on the page linked, as it doesn't require soda ash. Don't use RIT or drugstore brands. Your colors will not be deep enough and will fade quickly. You need a non-aluminum container big enough to have the garment float freely in. You can't reuse any utensils for food. You can do it in a washing machine, but it takes a lot more dye and careful attention. It can be messy, so protect all your surfaces and use gloves.
And take before and after pics!