Hello, this post sounds like it would be the first selfbow. Maybe the following helps:
Get a rough idea, of the bow design you plan to achieve. Whatever design (
longbow/flatbow?) you come up with, it will have less thickness towards the ends/tips than towards the middle. Start by taking away little by little, but not too much at a time and test it whether it starts bending a little (floor tiller). Important, only take away wood from the belly side (which is facing towards you when shooting the bow).
The width can also be adjusted later when you have a clear idea of the bow design.
For further tillering, it's basically: Take away some wood gently from the area where the bow is still too stiff.