There are two ways to create and save a Pit Strategy:
From the pre race menu you will see an 'Edit Pit Strategy' option or whilst you enter the pits during a race (The red part of the track on the map before the start line)
Once in there the AI will take over and you have the option to 'Edit Current (Default)'
on either of these options and then change anything and press
again to save and the achievement is yours.
Here's a brief video in a race