First, you lay the tile, then wait 24 hours to do the grouting. Instructions vary with the time of curing for the grout, but I adhere to this: The grout should not be used for 3 full days. This is the time for a good cure of the grout. The instructions usually say that the customer during these three day period should spray with a mister the grout lightly twice a day for those three days. I am sure that no one does this, BUT its stated on the instructions.
Then after the three day cure, you can then apply the silicon or grout sealant. This is a very important step in my opinion. But, this is your only chance to apply this. The grout has to be clean and new.