1) you are going to make mistakes--accept it, as that is how you learn. Most flubs are correctable.
2) For your first build, collect your parts then draw a fullsize plan with correct dimensions. You can use others plans, books, etc. but nothing beats doing it yourself to get a feel for how the gun will go together.
3) Get a good book: Buchele's "Recreating the American Longrifle" or Alexanders "Gunsmith of Grenville County" are recommended. As a college professor of mine said: "Read, Mark, and Inwardly Digest" the information.
4) Ask questions. The curmudgeons on this board are a crusty lot, but will give you plenty of well meant feedback.
5) Now start your build. Don't forget #4.