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