Here's a simple breakdown of the customizable product within the ScalePrint Designer & Variants admin panel and your Shopify store:
Go to Customizable Products into Product Designer.
Click on Add a new customization button which is in right upper side then it will open a new window of all of your shopify products.
Choose one of the product which needed to be customize from your product list.
After select your product it will open customizable options. Here you can customize your product as your needs.
Now choose "Set print area" option then it will open a pop-up option where you can provide your design by adjusting width or height of design.
In the last you can adjust design area on product then just click on "save" button for save to your design.
After saving it you can now see a "Go to Preview" option here you can preview it then now a "save" button visible click on it and save your design.
After clicking on save you can see a publish page click on publish button then it will open a popup which will show your product and ask again for publish click on that.
Finally you can a last pop up which is showing that "Product Published" then click on that "OK" . Product is published now.