1) Requires Far Harbor DLC, and no other mods.
2) Download which of the 4 mods you want, ie if you want legendary recon marine freefall legs, download ReconMarinefreefalllegs.
3) Simply drop mod file into programfiles > steam > steamapps > common > fallout 4 > Data.  Or use mod manager.
3) Remember to load mod
4) The same vendors that sell right and left legendary recon marine arms, now sell right and left legendary recon marine legs.