To find an ID for a resource in a Shopify store, start by opening the Shopify admin area. Then, navigate to the specific resource whose ID is needed. Lastly, look at the browser's address bar: the ID is the number at the very end.
This applies for most resources in Shopify, whether it's an order, or a location, or a variant. For example, when looking at an order, the URL will look something like this:
And in this example, the ID is still the number at the very end.