If you're asking how to trowel mortar when installing tile, realize that you are asking a fundamentally critical question. It's so critical that readers like you have made this CTEF Blog article the one with the highest readership since we originally published it on June 7, 2016.
You see, when thin-set mortar is incorrectly applied, the installation will many times fail. More likely than not, the tile will break - as you see in the image below.
Not a good situation for the customer and a terrible one for everyone involved in installing tile correctly.
Let's focus then on how to properly apply the mortar.