Apply the Thinset
Use a 1/4" x 1/4" notched trowel to spread thin set over the first area to be tiled. Hold the trowel at a 45-degree angle to spread the adhesive, then use the notched edge to leave grooves in the adhesive. These air pockets help the tile adhere better.
Note: Be careful not to spread too much thinset at one time; if it dries out, the tiles won't stick. To test the thinset, press it gently with a finger. If it isn't sticky enough to come off on your finger, it's not sticky enough to hold a tile.