The pppd program is normally started when Linux boots. A script typically provides configuration parameters including what serial port to use and whether a connection should timeout after a specified idle time. Additional script files are normally referenced by this script so the pppd program can provide features such as logging and running a script when a connection is made and another when it is broken. There are a number of issues since pppd can handle more than just IP traffic. Features such as IPX protocol support are beyond this article.
Some details are more common.
For example, the initial handshake protocol can be specified. User name, passwords and domain names are details that these protocols require, normally supplied by the ISP when an account is opened with them. These parameters are placed into script files handled by programs like linuxconf. All you need to do is provide the details. The pppd daemon runs in the background.