fusée framboise is a custom bare-bones Raspbian image that has a systemd Unit that loops modshipd.sh of fusee-launcher.
A Raspberry Pi Zero (any RPi works, but the Zero tends to run off of any power bank)
A power bank (for powering the RPi)
A non-powered USB hub (why?)
A USB-A to C cable (don't get a no-brand one: it may not have a needed resistor, and it may wreck your Switch or your USB source)
A USB-A to Micro cable (for RPi power)
A USB Micro to USB-A Female cable (only for the RPi Zero: for plugging in the USB hub)
Download a fusee-framboise image.
Flash it to your RPi's SD card. (If you don't have a flasher software installed, I recommend Etcher.)
Plug the USB Micro to USB-A Female cable into the RPi. (only for RPi Zero)
Plug in the USB hub into the USB-A Female cable.
Plug in the USB-A to C cable into the USB hub.
Plug in the USB-A to Micro cable into the USB hub, and into the RPi.
You should just be able to plug in your Switch, put it into RCM, and the payload should automatically launch.
Replacing the payload
To replace the payload, you have two methods: (both need sudo)
Replace fusee.bin in /etc/fusee-launcher/ with another payload. (recommended)
Copy another payload into /etc/fusee-launcher/, change modchipd.sh to point to it, and restart the fusee-launcher service, or reboot.
You can change the payload over the air. Here's how to do it: (your RPi needs wireless, and you'll need a mobile/normal hotspot to do this)
Set a root password with "sudo passwd", and take note of it.
Change your default pi user's password from "raspberry". (optional, recommended)
Turn on SSH: it's in Interfacing Options of raspi-confiig.
Configure your Pi to connect to your phone's network.
Reboot to connect to it, run ifconfig, and take note of the settings.
Set a static IP based on your ifconfig data.
Take note of your static IP. (you should probably take note of it on your phone)
Use an app that supports SFTP to connect to your RPi with the new root password. (iOS)