I think you will need to check some Native American resources on the net to find out about standard heights and floor dimensions for teepees. And then, using the diameter of the floor base as a guide, space the willows into a circle. As they grow taller beging to curve them towards the center, using stakes and ties to hold them in their curved position. As willows grow quickly I imagine it will be juts a couple of years or so before they reach high enough to be tied together above the cenr of the circle.

Trudi