You can mix the cassia, henna and amla together with distilled water and let it sit for dye release, about 8-12 hours at room temperature. When that is ready and you're all set to apply, mix buxus with warm water and then add the buxus paste to the henna/cassia/amla mix. I'd leave it on 1-3 hours. The longer you leave it on the browner it will be, most likely. Shorter time would make it redder. This is why a strand test is recommended

I haven't used shikakai and don't know if it matters when that is added.