making a crossover cable is both fun and tedious. It's up to you whether you buy one or make one. If you choose to make one, you'll need just a regular cat 5 network cable, and you'll also need to buy a crimping tool, and the rj-45 8 modular connectors.
As you can see, buying an already made one has its advantages.
regardless, you would just plug the cable into both machines, make sure both machines are on the same subnet, same workgroup, have valid user account on the XP machine, on the 98 machine you'll only be able to put a password on a directory.
after that, you can share and backup from one machine to another.