Great question! I've provided step-by-step instructions below.
1. Click on "Discounts" in your admin
2. Click "Create discount" in the upper right corner
3. Select "Discount Code"
4. Title your discount "Free Shipping"
5. Select "Free Shipping" under "Type"
6. Select which countries the discount applies to
8. Select "Minimum Purchase Amount" and enter in the value
9. Select which customers this discount applies to
10. Select if you wish to limit the discount to one per customer and/or how many times the discount can be used in total by all your customers
Once you've completed those steps, you can select your publish date and click save! Our help document here also dives into the discount area in further detail should you want to learn more.
How is the rest of your setup going? Are there any other questions I can help with?