For most ppp connections (doesn't use pap or chap), you're only
concerned with three files:

ppp-on
ppp-on-dialer
ppp-off

You can find template versions of these files in
/usr/doc/ppp-2.3.7/scripts (your ppp version number might be different).

put ppp-on and ppp-off in /usr/sbin put ppp-on-dialer in /etc/ppp

Now open ppp-on and fill in the information that is relevant to you. In
ppp-on you'll see a bunch of variables to set near the top. And at the
bottom where it does the exec /usr/sbin/pppd call, make sure it's
calling the proper device, and is defining the proper speed for your
modem. minicom is a good piece of software to figure out your modem 
settings (although I can't get minicom to work under R5).

Then in ppp-on-dialer make sure it fits your connection requirements.

You shouldn't need to touch ppp-off

Lastly, make sure all three files are set to executable!

call ppp-on when you want to connect, and ppp-off when you want to
disconnect.