There is no official documentation yet (“We need to add explanation about supporting the spec into keycloak-documentation”), but the “How to try it” (keycloak-community/oauth2-device-authorization-grant.md at master · keycloak/keycloak-community · GitHub) section in the doc gives decent instructions. Have you tried that? I went through it with Keycloak 13, and everything works. Let us know if you have specific questions.