Click on the hamburger menu icon next to your current theme and choose "Edit code" from the drop-down menu.
In the "Layout" folder, select your "theme.liquid" file and paste the JS Snippet in the <footer></footer> section.
<script type="text/javascript" src="https://cdn.seojuice.io/suggestions.v1.js" defer></script>
Do click on the "Save Changes" to publish your changes.
Now you can go to your Shopify Website and verify that SEOJuice script has been added by viewing a page that has been analyzed be SEOJuice and has links attached to it. They should be automatically replaced with your Keywords / Links.