HOW TO BUILD AN UNDERGROUND TREE FARM:

FIRST: start off with a simple building or structure

SECOND: dig down twenty blocks (or as far as you'd like) and make a 21x21 room (or larger). make the height of the room at least 7+ blocks high

THIRD: begin to decorate or model the room with blocks of your choosing (i go for simple stone as my wall and roof structures)

FOURTH: dig a 15x15 (or larger) hole two blocks down in the center of your room

FIFTH: fill your hole with water below the first block

SIXTH: start filling in your hole with blocks of your choosing above the water (i go for sandstone so i can tell the difference on how far to place my seedlings apart)
*ADVISED* DO NOT USE GRAVEL OR SAND, AS THEY APPEAR TO DROP BELOW INTO YOUR WATER SUPPLY

SEVENTH: begin placing dirt two blocks apart from each other in place of the sandstone and begin growing your trees!