Part of Join It's Starter Subscription allows for Organizations to create custom fields on the join/checkout form and associate this information with Membership records.
You can determine the order of the custom fields on the checkout page by the order that they are 'activated' for a specific Membership Type.
The best way to select your own order is to turn off all of the custom fields for a specific Membership Type.
Then, you can activate the custom field that you want to appear in the first position on the checkout form. Next, activate the custom field that you want to appear in the second position. And continue until you've activated all of your desired fields.
Finally, click 'Save Questions' and you're all set!