When you grout, make sure that you have the right tools for the job - a sponge with a mesh backing and a small pick. Start by cleaning the tiles with water and a mild detergent. Use the scrubbing side of a sponge like the one pictured below and be sure to clean any mortar left behind in the grout lines with the pick.
When you are finishing a shower niche, lay the final tiles in this way:
When installing large tiles like 12x24 on a ceiling, ensure that your thinset mortar is mixed to a thick consistency. Otherwise, you will need a hard hat as you can expect the tiles to fall on your head! In addition, make sure you set the tile with some serious upward pressure for a good 20 to 30 seconds. Visually, plan out your tile placement so that the joints on the ceiling tiles are offset from the joints on the wall.