My phone isnt currently supported by lineage and many other phones that either my friends or i have. So i would like to know how to add a new device tree and kernel for any self-builder to use out there.
First, you need to visit the Community page of LineageOS, which will guide you on how to contribute to LineageOS. If you are a developer, then this page will send you to Gerrit to do online code reviews about LineageOS.
Once you are familiar enough with how LineageOS works, you can actually go to GitHub and contribute until you are confident enough to ask to be a member of LineageOS' team.
Then, you can create a new repo under LineageOS' name which will use the codename of the device you want to add and start developing on it.