If you have followed the other parts of this tutorial, then the folders and files are already setup for packing into a vpk file.

Create the vpk file one of two ways, via "Send To" or via drag-and-drop.

Create vpk file via "Send To":

Go to the addon's Release folder.

Select the folder that has the name of the addon.

Right-click on the folder.

Select Send to > L4D2 VPK

OR

Create vpk file via drag-and-drop:

Go to the addon's Release folder.

Select the folder that has the name of the addon.

Drag the folder and drop it on the vpk.exe file.

When that finishes, you should find a vpk file with the same name as the folder.