Users are advised to read :-
Whilst you can use this document to create your PPP link without reading any of these documents, you will have a far better understanding of what is going on if you do so! You will also be able to address problems yourself (or at least ask more intelligent questions on the comp.os.linux... newsgroups).
These documents (as well as various others, including the relevant RFCs) provide additional and more detailed explanation than is possible in this HOWTO.
If you are connecting a LAN to the Internet using PPP, you will need to know a reasonable amount about TCP/IP networking. In addition to the documents above, you will find the O'Reilly books "TCP/IP Network Administration" and "Building Internet Firewalls" of considerable benefit!