After taking the waste kit apart you need to put the white plastic chamber and the black rubber washer on the under side of the main bowl using a silicone sealant on the surface between the top of the rubber washer and the underside of the bowl.
Also use silicone sealant on the under side of the metal flange going inside the bowl to provide a sufficient seal then using the centre bolt provided tighten it all up as tight as you can forcing any silicone sealant out and wipe the excess away.
We also recommend you use some sealant on the overflow to prevent any leaking from here as well. Do the same procedure for the half bowl if you have purchased a 1.5 bowl sink. If you have a singel bowl just follow the instructions above.