|This presumes you have already downloaded the XMuscle addon and have created a musculature.|
This presumes you have already created a mesh/body for the muscles to effect.
This presumes you have already created a bone armature to control the mesh/body and musculature.
Select a muscle
Go to N Sidebar -> scroll to bottom -> 'Select XMusculature'
Shift+LMB the body
N Sidebar -> Apply Muscles to Body
Shift+LMB the armature
Ctrl+P -> Automatic Weights