Compiling the PPP daemon software See the documentation that comes with the version of pppd you are using.
Connecting and configuring a modem to Linux (in detail) See the Serial-HOWTO and for modem specific initialization, see Modem Setup Information for information that may help you to configure your modem.
Using DIP to make PPP connections. Use chat instead...
Using socks or IP Masquerade. There are perfectly good documents already covering these two packages.
Using EQL to gang together two modems into a single PPP link. This is covered in the Networking-HOWTO.