Acorns and oaks
Get a lot of acorns. The fresher the better--even green.
Make sure that worms haven't bored a hole in them.
Put them in a bucket of water.
Throw away the ones that float.
Keep those that sink.
Letting them get cold
2x6" frame with chicken wire on the top to keep the squirrels away.
Get the whole taproot and a big root ball.
Early in spring before the leaves unfold or in the fall after the sapling is dormant.