I am in the midst of making a 17th century stool of green red oak (my first try a green wood building). Having rived out, chopped and planed the stiles, and rived and froed the rails, I will offer a couple of lessons learned:
Start with a straight 20"+ diameter log with the heart pretty well centered. I am trying to get enough straight rived wood from a 5' x 17" diameter red oak. Problem is that it had knots from branch growth, a pronounced twist, and an off center pith. With a log that small, you have little left of the good heart wood after you take off the sap wood and the juvenile growth in the center, so it is hard to get a 4" wide board.
Get another log (can be ugly but needs the ends parallel to each other) to use for a chopping block. The one I am using is about 14" diameter and about 20" tall (I am 5' 8"), and it works well.
You will need a froe break (something to hold the split log pieces when you are levering with the froe).
IF you haven't already visited them, a couple sites that have a lot of good info are:
www.greenwoodworking.com. .. Has some good info on using a froe, etc and working green wood.
pfollansbee.wordpress.com .. He makes 17th century furniture, and this is his blog. You need to scan through the last couple years worth (1 blog per month) to get a good handle on riving and the joinery tips.
Glad to see I'm not alone starting this journey, altho I am doing it without a lathe.
Go
PS. Scott is a stellar individual full of knowledge about trees, logs, and wood.
