Add the Size chart
Navigate to the Shopify Admin -> Settings -> Metafields and metaobjects -> Products and click on the “Add definition” button.

Create a metafield with the following parameters:
Name: Any preferred name (e.g., Size guide)
Key (Namespace and key) field:size_guide.page. Click the row under the “Name” field and rename it.
Type: One, and Page
You should get the following result:

Step 2: Create a page with the Size Guide chart
Navigate to the Shopify Admin -> Online store -> Pages and create a page with the necessary content. Make sure you have chosen visibility as Visible.
Note: You can create as many different pages as you require.

Step 3: Connect the Size Guide page to the metafield in the Product settings.
Navigate to the Product in your Shopify admin, scroll down to the metafields block, find the metafield you created earlier, and select the needed page for the metafield.
Note: Please repeat this step for each of your products where the size chart should be displayed. The size guide button will not be shown for products that have an empty metafield.

Navigate to the Theme editor -> Product template -> scroll down to the Size guide block settings and enable the Show size guide option.
You can also specify the option where the Size guide button will be placed. If this field is empty or the option name does not exist, the size guide button will appear under the options.

Add model image (Optional)
In the Size Guide window, you can also display additional model information, like a model picture and text. To do this, follow these steps:
Navigate to the Product metafields and create a metafield to store the model image with the following parameters.
Name: Size guide model image.
Key (Namespace and key): custom.size_guide_model_image – This step can be skipped if the name of this metafield is set to “Size guide model image.”
Type: One, Image (File)

Navigate to the Product in your Shopify admin, scroll down to the metafields block, find the metafield you created earlier, and add the needed model image for the metafield.
Note: Please repeat this step for each product where the model image should be displayed.
Add model description (optional).
Navigate to the Product metafields and create a metafield to store the model image with the following parameters.
Name: Size guide model info.
Key (Namespace and key): custom.size_guide_model_info – This step can be skipped if the name of this metafield is set to “Size guide model info.”
Type: One, Multi-line text

Navigate to the Product in your Shopify admin, scroll down to the metafields block, find the metafield you created earlier, and add the text description.
Note: Please repeat this step for each product where the model description should be displayed.