NOTE: I sorta swiped this from Kimmie's page for my own personal use. She gets most of the credit for the info compiled below. I just added a few things for clarification and threw in some color for easier understanding. If you want to check out her site (and I encourage you to do so) it can currently be found at http://novaone.dynip.com/xirtam/.

There are two ways you can set it up:

First of all you need to be using pppd 2.2.0F or higher, to check which pppd you are
using type pppd --help and it will give you your version.
Get a newer pppd

For a terminal chat script you need to do the following:

Rename your /etc/ppp/options to /etc/ppp/optionsold

mv /etc/ppp/options /etc/ppp/optionsold

then make your /etc/ppp/options files look like the following using your
user information NOTE: make sure to edit the /dev/cua0 line to match your
modem device and replace the username entry with your username:

I like pico, you may have another favorite editor
pico /etc/ppp/options

# /etc/ppp/options
#
# $Id: options,v 1.4 1996/05/01 18:57:04 alvar Exp $
#
# Originally created by Jim Knoble
# Modified for Debian by alvar Bray
# Modified for PPP Server setup by Christoph Lameter
# Use the command egrep -v '#|^ *$' /etc/ppp/options to quickly see what
# options are active in this file.
/dev/cua0 normally cua0=com1, cua1=com2, etc.
-detach
defaultroute
persist
crtscts
user
username ex: bubba
remotename
usit

ctrl-x to save and exit pico

Next you have to make a pap file it should look like the following with
your user information in it ie: replace username line with your username
and the password line with your password:

pico /etc/ppp/pap

username ex: bubba
password ex: g0vO1s!

ctrl-x to save and exit pico

Now for the pap-secrets file, it should look like the following with your
user information in it ie: replace username line with your username
and the password line with your password:

# Secrets for authentication using PAP
# client server secret IP addresses
username usit password
example: bubba usit g0vO1s!

Now you have to create a chat script file called ppp-go to be placed in
your /usr/sbin directory it should look like the following with your
dialin phone number instead of the number thats there if its not the same
and make it executable:

#!/bin/sh
/usr/sbin/pppd connect '/usr/sbin/chat -v "" ATDT2920331 CONNECT' &

chmod 755 ppp-go to set to root=rwx, group=rx, world=rx

NOTE: this file, ppp-go, is where you place the actual connection command.
You might want to do a man pppd to find out more info about the command line
parameters of pppd. I found that if you use the line given here, you won't get all
that great of a connection. At the bare minimum you'll probably want to specify a
connection speed.

Now in /etc/resolv.conf add the nameservers of your ISP (example below):

domain usit.net
nameserver 199.1.48.2

As a precaution I suggest making the files root access only,
/usr/sbin/ppp-go
/etc/ppp/options
/etc/ppp/pap
/etc/ppp/pap-secrets
/dev/cua0 (substitute your modem device)

The other way to setup ppp connections is via X:

Download ezPPP
Grab the static libs, the README and HOWTO's are very clearly written on
how to set it up.

Both ways work very well on Slackware & RedHat. To my knowledge they both
work well with several other *nix's also.

Good Luck and Happy Connections :)