Constructing a null modem cable is really quite simple. All you need
is an RS232 cable and a null modem adapter. These are cheap, and 
can be found at Radio Shack.

Before you buy this equipment, you must find out what kind of serial ports
your computers have. Some serial ports are 25-pin, some are 9-pin. If
one computer has a 9-pin port and the other a 25-pin port, you can 
buy a pin changer (also from Radio Shack, also cheap) to change one 
to match the port on the other computer. 

Probably the ports are male, but this should also be verified. Buy an 
RS232 cable, 25-pin or 9-pin, that is appropriate for your computers. 
Radio Shack sells null modem adapters that are both male and female, 
25-pin or 9-pin. Buy one appropriate for you computers. Attach the null
modem adapter to your RS232 cable. It probably doesn't matter which 
computer you attach to the end of cable with the adapter on it. And 
that 's it. The total cost should be under $15 with tax.

I have used this cable and adapter to successfully transfer data between
a Texas Instruments laptop running MS-DOS 6.0, and a 286 clone running 
DR DOS 6.0. It works with Microsoft's Interlink and DR DOS's Filelink.