Before doing anything, let's keep some important stuff in mind:
Of course, first of all you need to have the SDK imported into the project in order to upload your content. Never mix the SDK2 or SDK3 in the same project, do clean uninstalls of the sdk prior to uploading or migrating the projects as it can corrupt your data. Currently (22/June/20) the most stable SDK is the VRCSDK2 as it contains the legacy inputs and triggers for worlds.
As the official page says: "Choose wisely."
Worlds with cross-platform support share the same "seed" or "world-id" provided by VRChat once you upload the world for the first time. There is no real limitation for the PC version more than having a thought on lower pc machines and not to overload the scene with RT events or graphics that require real-time processing.
To start the port to the Quest version, first of all duplicate the project as a way of back up and also to make the convenient tweaks you think you have to do to match the hardware limitations of the platform. The Unity profiles is a great help to know what is loading and how much is taking to render each frame of the camera.
As a creator, you should keep an eye on the performance of your world:
For the Quest version (which is the version that is going to run on an Android platform with graphical limitations), keep in mind the following hard specifications:
Prepare Unity to be able to upload scenes to VRChat
Once you have this basic setup, you now can press the big button and upload your content for the first time!
Once the SDK packages all your data and makes it ready to upload, prior to start it will ask you to put the name and other descriptions to your world (Unity will go play mode). Do a thumbnail, state tags or mark specific content and check the box prior to upload.
If you want your World to be public you must have at least "Known User" to be able to send it to Community Labs and let VRChat review if your World is good to be Public.
My PC build is live, what now?
Next steps are important, first of all duplicate your project, this way one project (development ended) will be the PC version and the other will be the Quest/Android version. I usually make use of the PC version as the master and the Quest version a semi-automatic downgrade.
Once duplicated, open the project with Unity. We are going to need the project in Android, once its open, go to File > Build Settings. You will see a list of diferent platforms to build your project with. Click on the Android platform and next click "Switch Platform".
Click on Switch Platform and wait until the process ends. It may take some time, depending on the size of your project. Once ended the platform switch, the Unity environment will be set with Android.
With the change of environment, you will no longer be able to do local tests so keep in mind to do "core" stuff on the pc build prior to duplicate the project. Use this project as a downgrade on texture sizing, high level compression and low-near-zero dynamic or real-time events being displayed on screen. With the same scene, the descriptor will be mantaining its World ID and with the upload the World will be updated with Quest support.
During the upload keep all info input the same as the PC version: name, tag, description...
That's it, we hope to see your amazing world over VRChat!