Bcachefs is not yet upstream - you'll have to build a kernel to use it.

First, check out the bcachefs kernel and tools repositories:

git clone https://evilpiepirate.org/git/bcachefs.git
git clone https://evilpiepirate.org/git/bcachefs-tools.git

Build and install as usual - make sure you enable CONFIG_BCACHEFS_FS. Then, to format and mount a single device with the default options, run:

bcachefs format /dev/sda1
mount -t bcachefs /dev/sda1 /mnt

For a multi device filesystem, with sda1 caching sdb1:

bcachefs format /dev/sd[ab]1 \
    --foreground_target /dev/sda1 \
    --promote_target /dev/sda1 \
    --background_target /dev/sdb1
mount -t bcachefs /dev/sda1:/dev/sdb1 /mnt

This will configure the filesystem so that writes will be buffered to /dev/sda1 before being written back to /dev/sdb1 in the background, and that hot data will be promoted to /dev/sda1 for faster access.

See bcachefs format --help for more options.