How to install:
1. Go to your OpenMW install location
2. Go into the "resources" folder, make a backup of the entire "shaders" folder
3. Copy over the "shaders" folder from the download (pick the one according to your OpenMW version!), which will overwrite several existing files
4. Back to the OpenMW install location, then into the "resources" folder, then into "vfs", then "textures", then "omw"
5. Create a backup for "water_nm.png"
6. Copy over "water_nm.png" from the download folder, overwriting the default file
7. Ensure than in your settings.cfg (located in "My Games\OpenMW") the [Shaders] section contains these 3 lines:
    force per pixel lighting = true
    force shaders = true
    lighting method = shaders
8. Also in your settings.cfg, ensure that your [Water] section is like this:
    rain ripple detail = 2
    reflection detail = 0
    refraction = true
    rtt size = 2048
    shader = true
    sunlight scattering = true
    wobbly shores = true
9. If you have never used the PBR lighting shaders before, keep reading. Otherwise enjoy!

Depending on what kind of specular maps you are using, you might or you might not run into issues. This is mostly common when people use old-school specular RGB maps that will be used as PBR maps, even though they are not!
You can either use the conversion scripts (separate optional download) to convert your existing _spec.dds maps to the expected PBR format and be on your way, or change SPECULAR_MAP_INTERPRETATION in lighting.glsl (found in shaders/lib/light)
You cannot mix between actual Specular and PBR maps (and if you are using PBR maps, they have to be consistent, either all using roughness, or all using smoothness - you can also download the optional conversion scripts to convert all PBR maps to use the standard roughness, instead of smoothness)
If using Specular maps, then the code will estimate PBR parameters. This isn't perfect, and will never be, but it errs on the side of caution trying to not make things too shinny. Upside is that that there are many specular maps available for all kinds of content
If using PBR maps, then the code will directly load the PBR data from them. As long as the data in them is good (and roughness isn't inverted into smoothness!), then the results should be good. Downside is there aren't many PBR maps available, they aren't available for all objects, and the quality can be hit-or-miss
If using either Specular or PBR maps, for any missing textures the code will use the defaults from the shader, which you can change if you want. The numbers provided have been tested a lot, and shouldn't cause shininess where it doesn't belong

Quick troubleshooting:
If anything looks wrong, or aren't sure about things, you can set SPECULAR_MAP_INTERPRETATION to 0 (or just disable object and terrain specular maps) and the shader will use very safe and sane PBR values for everything!
If you're getting some error while OMW is loading up, you most probably did not overwrite the shaders folder, and instead simply used the one in the archive, so you're missing all the other shaders, and the game can't run. Read the instructions carefully!