https://imgur.com/ZPapkwj - this is basically the method. This comes from a cookbook called the Book of New Israeli Food. I use Pereg Shwarma Spice mix which comes from Israel https://www.pereg-gourmet.com/products/mixed-spices-shawarma but you can make it yourself
Michael Solomonov has a new book coming out in October which contains a recipe for Shwarma. The cookbook is aimed at home cooks who want to reproduce the flavours at home without a spit so it might be worthwhile checking it out when it's released.