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.
Arrange with the network administrator of the other LAN the IP numbers that will be used for each end of the PPP interface. If you are using static IP numbers, this will also probably require you to dial into a specific telephone number.
Now edit the appropriate /etc/ppp/options[.ttyXX] file - it's a good idea to have a specific modem and port at your end for this connection. This may well require you to change your /etc/ppp/options file - and create appropriate options.ttyXX files for any other connections!
Specify the IP numbers for your end of the PPP link in the appropriate options file exactly as shown above for static IP numbers.