Actually pretty simple.
Here's an example: https://parkercs.tech/enabling-802-11r-fast-roaming-transiti...
I did lose the central management interface of Mikrotik (CAPsMAN) but for my home set-up this wasn't a big deal. I used the backup and restore capability in OpenWRT's LUCI interface to clone most of the settings.
Roaming / fast transition now works much better. I do still lose a few packets as I wonder around but nowhere near as flakey as on the Mikrotik stack.
I did spend an inordinate amount of time optimising channels, signal strength and placement etc. on the Mikrotik stack before migrating over, and so I kept these settings on OpenWRT. I think a lot of making wifi work well is in this particular black art. All things being equal, though, OpenWRT works better for me.