Download the tar.bz2 archive onto a Raspberry Pi running Debian Stretch with kernel version 4.19.66 (check the line starting with VERSION_CODENAME= in /etc/os-release on the Raspberry Pi if not sure which Debian version is installed; the kernel version can be found by running cat /proc/version). See the Raspberry Pi guide for information about how to install the kernel package, and where to install the application binaries, the Lepton kernel module and the device tree overlay .dtbo file.
The kernel package was built from the raspberrypi-kernel_1.20190819-1 tag on https://github.com/raspberrypi/linux. Both it and the binaries were built using the current version of the Raspberry Pi OS "Stretch" toolchain.
Unpack the tar.bz2 onto the Raspberry Pi SD card with tar jxf lepton_raspios_stretch_20190819.tar.bz2 and cd into the unpacked directory to install the files.