Lash mapping allows you to know exactly which lengths you are placing, and where.
First you will decide on the longest lash length you are using on your client based on your clients natural lash length. You want to be using at least 3 different lengths of lashes throughout the lash set for the most natural look, and keep the inner corners short as those natural lashes are typically the shortest and weakest naturally.
Decide on the style that would best suit your clients wants and needs, and using a sharpie or pen, mark on the under eye pads which lash lengths you will use in each section of the lash line.
There are endless lash mapping styles and below some of the trendiest. Remember to pick a style and then adjust the lash sizes accordingly to stay within a safe weight and length for each individual lash line.