First install zip as you would for any new add on by clicking install and choosing the downloaded zip.

Have your mech designed or not. Click generate rig. Position the bones how you want them. It is useful when first starting off to look at the automatically generated bones names. This is important for when selecting the appropriate parts of the mesh to parent to.

Use the eye dropper to select the parts to parent to bones.

Use the dropdown menu to select the bones to parent to.

Click attach meshes to bones button

https://www.youtube.com/watch?v=7t70SJ4OG3A