Drag the lml folder and the three asi files to your RDR2 Directory (Where the RDR2.exe is), if you already have an lml folder, still do it, it'll just transfer the lml contents in this mod to your lml folder.
To make the 1903 work, you need to either pick the Tempered or Steel variant. Grab the ytd from the variant you chose, and put it inside the Stream folder, not the "stream", the "Stream", located in (lml/Munitions/Stream)
Enjoy!

For the metals merge, pick one of the available options on the page, and put the new pbrweaponsurfaces.ymt into the PBR folder (lml/Munitions/PBR)