Prime your wall before you apply your tiles and let the primer dry. If you skip priming, you need to use constructive adhesive to attach the tiles. Priming is faster and easier than applying construction adhesive to every single tile. I would recommend having your primer tinted to match the colors of your tiles. I made a couple of mistakes so there are TINY gaps in one or two spots. The white primer shows through. If the primer had been tinted, the mistake would have been impossible to see (in fairness, it's still pretty hard to see).