I assume that you've put the HD inside the amiga? right then, boot the amiga with your workbench 3.0/3.1 disk in the drive, you should then be able to format the new HD from one of the menus...I think...
once that is done, boot with the workbench install disk in the drive and follow the instructions (or download BloodWych's ClassicWB from download.abime.net since it looks very nice
just a quick note, for 3.0/3.1 amigas partitions must be under 2.0GB in size, so you'll need to have two partitions
sorry...I should be more helpful once I get my new HD sometime soon