Here are the steps to create a fitment bucket. Also This link has a video to walk you through how to create a Bucket.
1. Click “Buckets”
2. Click the “Bucket Name:” field and add a name.
3. Click “Add”
4. Then add the Make
5. You can also add the model, sub model, or year range.
6. Click “Add” you can add as many different fitments as you want.
7. Click “Save”
8. You’re done creating a fitment bucket! This should show up under “Collections” in your Shopify admin; at which point you can work with it just as you would any Collection in Shopify.