This is a hard muscle to stretch on its own so we often use “the stick” to help.
Sit down on a chair and grab the handles on the stick.
Place the middle of the stick on the meaty part of your muscle directly on the outside of the shin bone. Roll back and forth while loosening the muscle.
To add effectiveness, start with toes raised towards the knee (dorsal flexed) and slowly roll the stick on the muscle while pointing toes in the opposite direction.