It’s in the bulk section of my local co-op. I assume you can find it in the baking aisle at a large supermarket.
First, sift the rye flour. Some of the granules are large and coarse so discard them.
Then mix a couple tablespoons of the sifted rye flour with some warm water so it forms a watery paste.
Get your hair wet (cold or lukewarm water) and apply the paste to the root and just scrub it in. You shouldn’t have to do anything except the roots, but you can do the rest of your hair if you want.
Rinse thoroughly. I’ve read that some people notice flecks of rye flour in their hair as it dries, but that’s never happened to me.