Excerpt from http://www.stickbow.com/FEATURES/BOWBUILDING/tillering.CFM.
Tiller is the difference between the upper limb and lower limbs measurements from the bowstring to the belly at the fadeouts. Bows are traditionally tillered to produce a stiffer lower limb; ie, the bowstring to belly measurement on the lower limb is less than the same measurement on the upper limb.
Tillering is the adjustment of tiller by removing material from a bow limb or limbs. Most bows are tillered to approximately 1/8″ – 1/4″ positive measurement on the upper limb, when the bow is to be shot split fingered. I tiller my bows even for a 3-under shooter. The following picture shows two methods of measuring, one using a bow square and the other a clear plastic ruler. Make sure that you always measure perpendicular to the string for the greatest accuracy.
This is the simple tillering tree I made. I made it to be held in the vise so I can store it when I’m not using it.
The tillering string is braided bale string found laying in a hay field while hunting. End loops are just scrap leather.
A simple screw on the side holds the hook when not in use. I got tired of picking it up off the floor.
draw length is indicated. The slots were simply cut on the band saw.
I’m not sure it was actually required, but i padded the top with a scrap piece of leather.
I found an old pulley laying around so it became the fulcrum for the tillering
I hope it gives some ideas or some inspirations.