We're going to need a couple things before we can get started,
This step is for anyone who hasn't installed unity, if you already have it up and running you can skip to step 1.
Ensure you are installing VRChat's recommended version, other than that you can run through the setup without having to change any options.
Go ahead and create a new project, set to 3D, save it to where ever though I do recommend you have some sort of structure to store your projects.
After the project is open check the title bar to ensure it ends with
PC, Mac & Linux Standalone <DX11>, if it does not then go to
File > Build Settings... then select
PC, Mac & Linux Standalone then click
Switch Platform in the bottom left.
Now we need to import the SDK, find the SDK unity package where ever you downloaded it and open it, it should open an import dialog, click Import.
After Unity finishes importing the SDK check the Console window in unity to ensure there are no errors (click clear first), if there are any errors search the error in a search engine of your choice or post a message in the #development channel in the VRChat discord
To use the SDK you will need to log in, to do so navigate to
VRChat SDK > Settings, there you can log in.*
* You require a VRChat account to upload custom avatar's and world's