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Preparation

Update:

apt update
apt upgrade
apt autoremove

Purge everything not required:

apt purge triggerhappy bluez wpasupplicant modemmanager avahi-daemon
apt autoremove

Disable Bluetooth and WiFi at boot, put these lines in /boot/firmware/config.txt:

dtoverlay=disable-wifi
dtoverlay=disable-bt

Install build chain and kernel headers:

apt install build-essential raspberrypi-kernel-headers git

GPIO26 is red, GPIO21 is blue:

Blue on:

pinctrl set 21 op dh

Blue off:

pinctrl set 21 op dl

Red on:

pinctrl set 26 op dh

Red off:

pinctrl set 26 op dl