Step 1: Fill 2/3 of the bag with regular potting soil. This soil is drier and of lower quality than the forest soil.
Step 2: Fill the bag almost to the top with forest soil. This soil will help the tree grow in its first year. The first year is the hardest, so it is important that it have enough nutrients. So bonus points if you add in some mycelium or roots.
Step 3: Put the acorn on top of the forest soil.
Step 4: Cover the acorn with regular potting soil for protection. Your seed bag is now done!