Basic instructions by Thomas Leister can be found here. Several Debian releases out of date, but mostly still sound.

To run on Raspberry Pi, the most useful source for a package seems to be this repository.

“It would be easier to run rspamd in a VM or a container, and I'd be a lot more confident of a successful outcome” G.W. Haywood rspamd@jubileegroup.co.uk (Ged) on the rspamd mailing list.