With the few sinew-backed bows I've made, I've done pretty much like Susquehannock said. Get it bending good and evenly, somewhere between floor tiller and brace-height. If you put it on when the stave is still stiff or bending unevenly, the sinew can't pull the bow into even reflex. If you wait too long, you're defeating the purpose of sinewing it to begin with, as the typical sinewed bow is too short or too stressed to hold up well without the sinew.