Easy answer, put the first disk in the floppy, boot on it and follow instructions on screen
Less easy answer:
- Try first to make a backup of all you personal data, even of everything if you can.
You will NEVER EVER regret having backup more than needed, on the other hand.....
CHECK that the backup are working before erasing anything!!
- find the drivers for all your hardware.
Most important are the graphic card (working in 640x480 is annoying and some modern card does not work well with Win98 basic VGA driver), AND AND network/modem card.
If you network are working you will be able to find the missing driver and/or help on the internet.
- Any special hardware that is needed to start the computer (special disk drive, raid drivers,special mouse, keyboard,...). The rest is less important (sound,..), if needed you can find them on internet after install
- notes the parameters for your ISP, have all passwords handy (you have a backup of these, don't you....
- check any program installed that need a licence, if you have the licence and original disks, or check with the company how to move the programme from computer. Some shareware for instance, need a key that is based on the serial number of the program and if you have downloaded it from internet, you may end up with another serial number, see how you can keep your licence.
- have a look at the list of hardware installed, and software installed. Write/ print it.
So you can re-install everything afterwards and don't realized the day you need it right this minute, that you didn't re-install program X or not often used hardware Y.
You can do most of it even if your machine is not booting anymore by booting on the first disk of Win98 by refusing the install to go to DOS mode.
Hopes it helps