You should need three floppies at least for Slackware 11, two with the install.1 and install.2 images written to them and one with your default kernel (bare.i). If that doesn't work then its possible that you have a bad ISO.
Put the disk that you wrote install.1 to in your floppy drive, restart your computer (with first boot set to A: of course) and follow the prompts.

Good luck.