Hey!
Here's what you need to do:
Install HIDtoVPAD normally
Get a configuration file, and put it in SD:/wiiu/controller/ (example: SD:/wiiu/controller/8bitdo.ini)
Open HIDtoVPAD
Choose the controller you want to emulate -- but if you choose anything other than the gamepad, you actually need one of the controller and need to connect it. (make sure you have a configuration file for the controller you want to emulate if you have something like the 8bitdo which isn't immediately supported when you connect it.)
Now, if you
a) have a USB controller or whatever it is, just plug it into the wiiu, choose the controller on the gamepad and click A.
b) have a bluetooth controller, go to the about section in FTPiiU and find the IP address. download the network client (I just googled it to download) and connect to the IP address shown on the wii u. Then, choose the controller you want to emulate on the gamepad and click A.
After that, if you want to add any more controllers, add them.
Now, just click + on any screen to apply the patches. This will take you to the Wii U home menu. If the controllers don't work, click the TV button.
Remember, you cannot run any other homebrew at the same time. (but I THINK loadiine has the thing built in... not sure though.)
(the network client also works for xInput controllers)