To prepare the styrene tops for gluing, you need to roughen the surface of the plastic. This is so the glue will have a better surface to stick to. Just take your sandpaper and lightly sand the side of the plastic without the film. Make sure you scratch up the whole of the surface . After remove any dust with a cloth.
To remove any grease or oils wipe the surface with a lint free cloth and some methylated spirits. Don't use lots of the methylated spirts pour just enough on the cloth to wipe down the surface.
The EVA foam tiles I bought have a pattern embossed (non slip?) on one side and are smooth on the other. I glued the tops to the smooth side as I thought there would be more contact surface for the tops to stick to.
The foam tiles should have some masking tape placed over the knobs along two adjacent edges on the smooth side. This will stop the knobs from being covered in glue. I didn't have any masking tape at the time so I used some 1 inch packing tape.