Documents to read are;
/usr/src/linux/Documentation/networking/PLIP.txt
/usr/doc/Linux-mini-HOWTOs/PLIP (Thats slackware)
I belive Redhat is something like;
/usr/doc/HOWTOS/mini (if installed of corse.)

Basicly all that is needed is the following commands "providing" one
is using a kernel with PLIP support compiled as a module or compiled
into the kernel. It is by far the preferred method to have PLIP compiled
as a module.

A small note for kernel modules and modprobe.
For all modules to work as they should via modprobe aliases need to be
placed into /etc/modules.conf or /etc/conf.modules (whichever your
version of modprobe/insmod uses).

• Example modules.conf

Note the last 2 lines of the modules file, they are important.

modprobe plip
ifconfig plip0 10.0.0.1 pointopoint 10.0.0.2 netmask 255.255.255.0 up
route add 10.0.0.2 plip0

Of course the other side of the link is 10.0.0.2 meaning just change
the IP#'s around on the other side of the link.
As in.
ifconfig plip0 10.0.0.2 pointopoint 10.0.0.1 netmask 255.255.255.0 up
route add 10.0.0.1 plip0
Thats it.
If you have any troubles consult the documents named above.