There is basically no difference between linking a single Linux PC to a PPP server and linking two LANs using PPP on a machine on each LAN. Remember, PPP is a peer to peer protocol.
However, you DEFINITELY need to understand about how routing is established. Read the NET-2 howto and the Linux Network Administrator Guide (NAG). You will also find TCP/IP Network Administration (published by O'Reilly and Assoc - ISBN 0-937175-82-X) to be of invaluable assistance.
If you are going to be sub networking an IP network number on either side of the link, you will also find the Linux (draft) sub networking mini-howto) to be of use. This is available at Linux Sub networking mini-HOWTO.
In order to link two LANs, you must be using different IP network numbers (or subnets of the same network number) and you will need to use static IP numbers - or use IP masquerade. If you want to use IP masquerade, see the IP masquerade mini-howto for instructions on setting that up.