If you'd like a completely customised size guide, you can create one externally and upload it as an image. To do this you need to upload it to get an image URL.
1) In your store admin menu navigate to Online Store > Pages and click 'Add Page'.
Name this page something like 'Size Guide' for future reference
2) Click 'Insert Image' and upload your size guide.
3) Choose your image and ensure the size is big enough for your customers to see it clearly
4) Click 'Save'
5) From the Pages dashboard, click 'View' on the size guides page you have just created.
6) Right-click on the image and click 'Open Image in New Tab'
7) Copy the image URL from the search bar
8) Navigate to the Size Chart Editor in your app and paste this image URL into the data field.
Ensure there is only one data field in your chart and that you have used the button toggle to the left of it to set this field as an 'image URL' field.
