Hi @Shankeyisgem.
In order to setup Selenium grid for Appium tests you need to have Selenium Hub running and 2 nodes,1 with Android capabilities and another with IOS capabilities registered with the hub.
Now if you are having your testsuite specified in TestNG.xml then you can specify parameter say e.g. "IOSTest" for a test which you want to give to IOS device to run and you can specify parameter e.g."AndroidTest" for a test which you want to give to Android device to run.
These settings "IOSTest" and "AndroidTest" based on which test distribution is going to happen,should be present in BrowserName capability included in .jason file which you may be using to start and register nodes.
And at script side you will be receiving these TestNG parameters and using it to set BrowserName DesiredCapability while instantiating DesiredCapabilities object which you will eventually use to create Driver object.
Note that to distribute tests among the registered nodes,"BrowserName" is the capability which Selenium Grid takes into consideration.
Hope this will help you out.If you want any other details then please feel free to contact me.
Thanks and Regards,
Pranoday