Note: if you have not done so, please read the introduction to Upgrading a PS3 Hard Drive before performing these steps.
It is important that you disconnect all cables from the PS3, including video cables, controller cables, other accessory cables, and especially the power cable. Now move the PS3 console onto a work area you have prepared and place it on its side as shown in the picture below. There is an HDD sticker on the one side, this side should be up.
Right by that HDD sticker is the plastic HDD cover plate, this can be removed easily with a flat tip screwdriver, or by simply using your finger nail to pry it up and off. Move on to the next step.