If you are using a manual shifting hub, you will need to install everything in order to control shifting. I'll provide the links to the videos for the hub that I used but this won't be universal to every hub.

Installing the interface

Setting cable housing length

To install the shifter on the handle bars, just wiggle and pull the grips off. I then had to loosen (there is a hex bolt on the under side of the brake lever, behind a wire coming out of the boost button) the brake lever and move that a bit further in. I still wasn't able to fit my 90mm grip on so I think I'll need to cut it down to size.

You will also need to install the shifter cable housing. I was able to use regular wire cutting pliers to cut the cable housing but it was very difficult. The designated pliers might be worth it if you don't already have wire cutting pliers.

This park tool video might be helpful for installing the housing

If you are using the enviolo hub and manual interface, make sure the housing ends so that it sits inside the manual hub interface

The enviolo hub and manual interface that I used had a issue here it was running into the chain guard so I had to unscrew the plastic chain guard. I also ended up needing to slightly bend the pictured metal piece out of the way of the shifter. You won't need to do this if you get hub with a smaller flange diameter