• Right-click the game in your library:
  
• Click "Properties"
  
• In the dialog that appears, click "Local Files"
  
• Click "Browse"

Well, duh. If you want to use them, the game needs to load them from somewhere

GOTCHA: The game does not support loading PODs from inside archives (ZIP, RAR, etc) - the POD must be in the game's directory as is.

Back in the day, we had a tool named RayneGun that was used to simplify mounting custom PODs into the game, but it no longer works on Windows 7 or newer versions.
You'll have to edit PCPOD.INI manually instead (which fortunately isn't very difficult)

• Find PCPOD.INI:
  
  
• Open it.
  
• If your machine does not have any program associated with the INI extension: it's just a text file, choose Notepad or any other text editor you like (personally I use Notepad++)

Now that you've found PCPOD.INI, you can see that file consists of two things:
1. A number on the first line. This tells the game how many PODs to load. (for an unmodified Terminal Cut, that will be 7; for unmodified Legacy version, that will be 6)
2. Several lines with filenames of POD files to mount when the game starts.

To add a mod, increase the number in the first line by 1 and add a new line with its' filename on the second spot:

IMPORTANT:

• If the number in your file is less than the number of POD files specified in PCPOD.INI, WORLD.POD won't load and the game will crash whenever loading a level.
  
• If the number in your file is larger than the number of POD files specified in PCPOD.INI, the game will crash on startup.
  
• If the filename is not exact, the game will crash on startup.

You can repeat this for any number of mods:

IMPORTANT: If you have two mods that replace the same asset, the file higher on the list will take precedence before files lower on the list.

GOTCHA: PCVOX.POD and the language PODs (ENGLISH.POD etc) are loaded separately and cannot be modified using PCPOD.INI

PROTIP: The game supports loading your PODs from a subdirectory (eg: Skins\SusanFlint.pod ). This could be useful if you want to have a bunch of mods to choose from but keep them at least a little in order.

Note: As a rule of thumb, mods that were compatible with the original release of Bloodrayne are going to work fine with Terminal Cut. Exceptions can occur if something replaced PCENGVOX or modified the pre-baked lighting textures, but I don't recall any mods making use of that.

• Some of the mods have been uploaded to Nexus Mods: https://www.nexusmods.com/bloodrayneterminalcut
  
• Several archives of mods (including my own) have been shared in a GOG thread about the Legacy release of the game: https://www.gog.com/forum/bloodrayne_legacy_series/are_there_any_mods_at_all/page1
  
• A handful of mods can be found in the Wayback Machine copy of bloodrayne2.ru: http://web.archive.org/web/20120212074655/http://www.bloodrayne2.ru/en/resources/files/skins.html
  
• Three skins can be found on the OOCities archive of one of the fans' personal sites: https://www.oocities.org/bloodrayneskinz/
  
• The DarkRayne skin has been uploaded to ModDB : https://www.moddb.com/games/bloodrayne/mods

Unfortunately, this handful of sources is "it" as far as mods for BloodRayne / BloodRayne Terminal Cut are concerned.
The game has been published close to twenty years ago, and the handful of fansites (Bloodrayne Massacre - later known as Game Vixen Zone - SparedLife's mod page, Bloodrayne.co.uk forums etc) have all gone out since then.
Crippled versions of the websites themselves can be browsed through The Wayback Machine, but none of the downloads work - I checked.