You can create the LVM setup with pvcreate, vgcreate, lvcreate, and lvconvert as usual.

Here is some copy paste snippet:

Setting up the LVM cache

First, we create physical volumes for our devices, and check that it worked
correctly:

sudo pvcreate ${VOLUME} ${DEVA} ${DEVB}
sudo pvs

Then, we create a volume group with our three physical devices. In that volume
group, we create three logical volumes: one for the data, another for the cache
and the last one for the cache's metadata. Then we check that everything worked
as expected.

sudo vgcreate cache-layer-vg ${VOLUME} ${DEVA} ${DEVB}
sudo lvcreate -l 100%FREE -n data cache-layer-vg ${VOLUME}
sudo lvcreate -L 700G -n cachedisk cache-layer-vg
sudo lvcreate -L 16G -n metadisk cache-layer-vg
sudo lvs

Now we need to turn those logical volumes into the actual cache. First we
attach the metadata to the cache and then the cache to the data.

sudo lvconvert --type=cache-pool /dev/cache-layer-vg/cachedisk --poolmetadata  /dev/cache-layer-vg/metadisk
sudo lvconvert --type cache /dev/cache-layer-vg/data --cachepool /dev/cache-layer-vg/cachedisk
sudo lvs

With that, we've set up the data volume to be cached with our SSD disks. We can
now format it and mount it:

sudo mkfs.ext4 /dev/cache-layer-vg/data
sudo mkdir /var/www
sudo mount /dev/cache-layer-vg/data /var/www

Would be nice to have a docs PR.

PR: flatcar-archive/flatcar-docs#315

Addressed in flatcar/flatcar-website#362 - available here: https://www.flatcar.org/docs/latest/setup/storage/lvm/