My advise: Before you actually start building (parts of) BLFS take the time to write down which major components you want to install. Once you have those use the book to find out which required (and possibly optional) dependencies need to be installed.
If, for example you want OpenSSH and ProFTPD you end up with the following list (parts in italic are just comments):
OpenSSL (required by openssh)
Sysstat (optional for openssh - entropy)
Berkeley DB (optional for linux-pam)
Linux-PAM (optional for openssh and proftpd)
If you do this for all the major components you want to install you'll end up with a long ordered list which can be used top down. This does take time and can be a bit of a puzzle, but it is worth it in the end. I would advise to make 2 lists: One that is CLI related and one that is X related.
Experience, as already mentioned by spiky0011, does come in handy
Hope this helps.