This is a great article posted on another site (who reposted it from another site, who took it from another), but I thought many would enjoy it here. It is The Lazy Man’s Guide for Constructing a Call of the Cthulhu Adventure, written by Sandy Petersen, original author of the Call of Cthulhu.
The greatest tree in the woods has a secret, there is a dungeon under it made from the intricate web of roots. What creatures could reside here? What secrets will the PCs uncover?