There’s now an official 64bit image, but it has a few issues still.

Official Image:

• https://downloads.raspberrypi.org/raspios_arm64/images/raspios_arm64-2020-05-28/2020-05-27-raspios-buster-arm64.zip
• https://www.raspberrypi.org/forums/viewtopic.php?p=1668160

Pre-Official Image Notes

The debian way works great on 3B+ systems, but leaves you with debian and not raspbian. The raspbian way leaves you with 32bit raspbian (there isn’t a 64bit raspbian), but running a 64bit kernel.

As close to official debian as one can get: https://salsa.debian.org/raspi-team/image-specs

Alpine modifications: https://raspberrypi.stackexchange.com/questions/100608/booting-alpine-linux-on-raspberry-pi-4

Old Raspbian way: https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=250730

• Get a testing kernel with sudo rpi-update
• Activate the aarch64 kernel with printf "arm64_bit=1\nkernel=kernel8.img\n" | sudo tee -a /boot/config.txt

Current Raspbian way: Just activate the kernel8.img kernel:

echo kernel=kernel8.img | sudo tee -a /boot/config.txt

If on a raspberry pi 4, also activate 64bit mode:

echo arm64_bit=1 | sudo tee -a /boot/config.txt