Note: Do not create another checkout.liquid. If you already have it, use it. And contact Shopify support if you do not already have a checkout.liquid and you're on Shopify Plus.
1. Contact your Merchant Success Manager, they can turn on the checkout.liquid portion of the site for you.
2. Head to the themes portion of your Shopify platform and hit edit HTML/CSS
3. Hit “add new layout”
4. In the drop-down menu, you will find a “checkout” option. Add that file and it will create a checkout.liquid template for you.
5. Scroll to the bottom of that file and paste in the Justuno embed code, here is an article helping you find that. Justuno will now work on your checkout page.
Note: The script embed code should be pasted in the footer or below closing body tags.