As you could probably guess, you need the following in order to go through with this guide:

• TFA-VOX || Player Callouts Redefined by TFA
  
• Enhanced PlayerModel Selector by LibertyForce
  
• A TFA-VOX pack (the one used in the guide is Combine VOX Pack by TFA-WS1)
  
• A player model that doesn't have an existing TFA-VOX pack (the one used in the guide is from Half-Life 2: Survivor Player Models by LOL DUDE)

Once you have all of those, continue on.

First, open up the player model menu (hold C and click Player Model in the top left), then click on the player model you want to assign a TFA-VOX pack to. You do not have to apply the player model.

From there, click the tab in the top right of the menu called Settings. This tab is not present in the normal Garry's Mod player model selector.

Now, click the newly-revealed tab named VOX.

Look down to the list named Available VOX packs. This is a list of every single TFA-VOX pack that you own, regardless of whether or not you have the player models they're usually associated with. Simply click on the pack that you want to assign to your chosen player model, making sure that it's highlighted.

As you could probably guess, all you have to do now is click the big button that says Assign VOX pack to current PlayerModel.

If you see the name of the player model you selected next to the name of the TFA-VOX pack you selected in the upper list, then you have successfully given that player model that TFA-VOX pack. The change should apply instantly, even if you're using the player model at that time. Conversely, if you'd like to remove a manually-assigned TFA-VOX pack, just click Remove selected assignment whilst you have the assignment in the list above selected.