How to add Boomer Vomit to L4D2 survivor models

In L4D2, when the survivors get attacked by a Boomer, their player model will be covered in vomit. This effect can be recreated in Source Filmmaker to make it look as if the survivors have been vomited on.

After spending a lot of time trying to find a way of creating this effect, I found this forum post from almost a decade ago that explains the process.
https://steamcommunity.com/app/1840/discussions/0/2906376154329629086/

I'm creating a guide for it to improve visibility of the solution and provide a graphical walk through to aide users.

Note: whilst I'm using my own version of the L4D2 survivor models in this guide, the process should be the same for other versions of the models

Start by spawning a L4D survivor model. I will be using Ellis.

In the Model Viewer, right click on the survivor model. Hover over the "Set Skin" option, and you'll see that it can be set to either 0 or 1. To apply the Boomer vomit texture, we need to use skin 1.

Now select the "Add Override Materials" option. This lets us edit the texture properties of the model.

View the model in the element viewer by selecting "Show in Element Viewer" > "Model", and then navigating to the Element Viewer tab.

In the element viewer, we are given options to edit the properties and textures of the model.

Expanding the "materials" tab gives us a list of all the textures that our survivor model is using. Because I am using Ellis's model, his textures start with "mechanic_".

Internally in L4D2, the 'vomit' status is referred to as being "IT". The vomit related textures end with "_it". We will need to edit the properties of these textures to make them visible. There should be 2 _it textures, one for the head and one for the body.

Right click the _it texture and select "Add Attribute" > "float". Name your attribute '$ITAmount'. This attribute controls how "it" the survivor is, IE: how much vomit they have on them.

Repeat this for the other _it texture. You should now have added two attributes called '$ITAmount'. Set the value of both of these to 1 for maximum vomit.

Our survivor model is now covered in vomit.