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.
There are some tillering videos ( usually several short vids following the progress of a specific bow) on my Youtube channel and blog (Bowyers Diary).
Personally I tend to move out from the centre... BUT this only works if you pull to full target draw weight from the beginning.
The fundamental principle is , always pull to full draw weight , as long as the tiller is reasonable... if any sign of a problem stop pullin gand remove wood where needed to improve the tiller.
I just posted a video of a Yew primitive I'm working on today... first flex on the tiller, having been roughed out to an approximate taper by eye.