Once the usable length of tree is back at the shop, start splitting it up. The part you're interested in is the white sapwood. Split off the heart as close as you can to the dividing line between the light and dark wood. You don't really need to do it, but I always shave off the bark and any remaining heart. I work it down until I have a piece of clean sapwood that is as wide in the plane of the growth rings as I want the splits to be—say, i inch for chair bottoms—and as thick as the sapwood was on the tree.
Now comes the proof of the matter. Take the blade of a jackknife and set it in the exact middle of the end of the piece in the same line as the growth rings. Drive the blade in with a stick to start the split. Once the split is started, work it down the rest of the way with your hands. Keep the split centered by putting more bend into the thicker side. This bend will further stress and weaken the fibers on the thick side and make the split run back that way.
Continue splitting each piece in half, with the growth rings, until they are as thin as you want or can achieve. Don't give up if it doesn't work at first. Quite often the wood on one side of a tree will not work at all, while that on the other side does just fine. There is almost as much variation within a single tree as there is between different trees. If you can't split all the wood right away, leave the log in a shady spot or even in a pond until you can get back to it. I've made excellent splits from logs that lay on the north side of the shop for as long as three months.
When you finish preparing the splits, gather them up and set them aside to dry. If you were to weave them now, they would shrink in width so much upon drying that even the tightest work would loosen up.
After the splits have dried a bit, you can shave them smoother by drawing them between your knee and a knife held vertical to their surface. If the splits are wider than you want, you can cut them to any width with a pair of regular scissors.
When the time comes to use them, don't soak them in water or you will get the same kind of shrinking problem. All you need to do is dip them for a minute or so; then they do quite well.
"The Woodwright's Companion: Exploring Traditional Woodcraft" By Roy Underhill
© 2012 The University of North Carolina Press