I think that any guides you find for Gitea + Renovate should work still for Forgejo + Renovate.
I believe the process is:
Create Forgejo instance
Create a user for Renovate within Forgejo
Using the CLI on your local machine (or another tool to complete this step), create an SSH public/private key for the Renovate user
Log into Forgejo using the Renovate user and configure the previously created SSH keys and separately generate a Forgejo token
Create a Renovate instance with settings for at least RENOVATE_GIT_PRIVATE_KEY (SSH private key value), RENOVATE_TOKEN (Forgejo token value), RENOVATE_PLATFORM (gitea), RENOVATE_ENDPOINT (Forgejo API base URL), and any other Renovate settings that you may find helpful/necessary to configure (eg: GITHUB_COM_TOKEN, RENOVATE_AUTODISCOVER, etc.)
Depending on how you want things to work, you may need to give the Renovate Forgejo user access to individual repos
I think that any guides you find for Gitea + Renovate should work still for Forgejo + Renovate.
I believe the process is:
RENOVATE_GIT_PRIVATE_KEY(SSH private key value),RENOVATE_TOKEN(Forgejo token value),RENOVATE_PLATFORM(gitea),RENOVATE_ENDPOINT(Forgejo API base URL), and any other Renovate settings that you may find helpful/necessary to configure (eg:GITHUB_COM_TOKEN,RENOVATE_AUTODISCOVER, etc.)