How do these interlock? Do you need anything additional to lock them together?

The interlocking of our Play Pad 5 is very easy. The interlocking pattern of this underpad is a brick pattern. The middle notch is large enough to interlock the top or bottom or two other Play Pads. No glue or tape is used to keep them in place. Once you have achieved your desired size, you would just cut the pads to fit.