Summarize this article:

Spread mortar in a small area where you will begin to work. Only work with a roughly 2'x3' area at a time. You do not want the mortar to have time to set before you can lay your tile. Using a notched trowel (different sizes may be needed, 3/8" is a good starting point), spread the mortar between sections that you marked with the chalk line.  If the mortar rises up between the tiles (to be flush or nearly flush with the tile surface), that means it's too thick or that the ridges need to be shorter. The mortar should be covering the entire tile, if you lift it up after placing it. If when you lift up the tiles you see only lines of mortar on the tile, then the mortar has dried out too much or the bed is too thin and the height of the ridges must be increased. If using tile sheets, use a trowel with smaller notches. This will keep the mortar from coming up through the gaps between the tiles. Lay your tiles onto the mortar, starting at the right corner you marked and following a straight line. Leave gaps of 1/8" where the edge of the tile meets up with the wall or floor. This is to allow room for expansion and movement, as the material naturally changes with its environment. This gap can be covered with grout, molding, or shoe tile. Place tile spaces between each tile as you go, or simply use your eyes to estimate if using tile sheets. These spacers are usually places at the corners of each tile and look like the cross shape that is formed by four tiles being next to each other. Use a carpenter's level as you go to make sure that the tiles are level. Use a masonry wet saw to cut any tiles you need for the corners and edges, carefully measuring them to fit for your particular project. Don't forget to leave the 1/8" gap all around the edge. Remove the spaces once the mortar has set and you're ready to grout!

Summary:
Spread your mortar. Lay your tiles. Insert the spacers as you go. Level the tiles as you go. Cut tiles for the edges. Remove your spacers before doing the grout.