There is nothing wrong with following a tutorial before you implement something (as long as you don't just copy and paste the code from the tutorial). The best way to approach any programming problem before you start hacking code together is to research the problem first. This may involve reading a book a journal, an online tutorial or planning it out on paper first.
Its only after repeating something many times that you store it as useful knowledge. So unless you are rewriting your A* every day although you can remember the gist of it such as the open closed list you may need to refresh yourself by following a tutorial before you remember the whole thing.