There needs to be a sticky or something about drying because this comes up a lot.
Rough those saplings out so they are bending a good bit at floor tiller. Dont narrow them at all to lessen the likelihood of warping. Notice i said lessen the likelihood. Some wood warps, some doesnt. The only way to guarantee it doesnt is to strap the wood down to something or put some weight on the wood. Set the roughed out peices inside where the humidity isnt too high. Depending on the humidity, they can be dry in less than a month or even 2 weeks.