Spread the Mortar
Apply the mortar to an area of the floor, along your chalk guideline. Make sure you leave the chalk line exposed so you can use it as a guide. Use the flat side of the trowel to spread the thinset mortar on the floor (Image 1), then turn the trowel around and double back over the thinset with the notched side of the trowel (Image 2).
Don’t apply too much mortar to the floor at one time, as it will begin to set up within 20 minutes. Just work a small area at a time, spreading the mortar out and installing a few tiles before moving to the next area (Image 3). If you do have to stop for any reason after you’ve already spread the thinset down, scrape it back up and put it in the bucket. It is much easier to get off the floor while it’s wet than once it’s cured.
Typically for floor installations, a 1/4” square-notched trowel is the right tool for the job, but if your floor is rolling, uneven, or if you are working with very large tiles, it helps to use a trowel with larger notches. Recommended sizes are 3/8” and 1/2”, which are easy to find in most hardware stores.