How can I dye white fabric curtains ombre style?

Start with very water downed color first. Dip it to the point you want it to start, the keep getting darker with each dye.

Emy has a great tutorial here:

https://www.hometalk.com/diy/living-room/curtains/anthropologie-inspired-ombre-curtains-with-fringe-38465072

Also make sure the curtains are made of fabric that will take the dye, like cotton

https://www.youtube.com/results?search_query=diy+ombre+dip+dying+curtains That's why we're proud to introduce Rit DyeMore, a dye specially made for synthetic fabrics like polyester, polyester cotton blends, acrylic, and acetate. ... If you are dyeing a garment with less than 35% synthetic fibers, use Rit All-Purpose Dye instead.

I used indigo dye to ombré dye a cotton shirt https://m.youtube.com/watch?v=TX7P0RjjG-g

Here's one of m favorite tutorials for this: https://www.remodelaholic.com/make-ombre-dip-dye-curtains/