Remember, if it says DOS, you boot from a DOS floppy - don't use a DOS session in Windows.
If it says disable DMA - DO IT!
In general, you DO NOT want anything in the drive - unless it specifically tells you otherwise, remove any disc that is in the drive before flashing.
And make sure you hit the right drive - of all the embarrassing errors that can be made, flashing the wrong drive is about the worst.
And finally, don't turn off or interrupt until it tells you - and don't start if the power is a bit iffy, if there are thunderstorms in the area which may cause a power failure.