You will need to edit the code in the cart.liquid template. If you're not familiar with the HTML/CSS theme editor, I would strongly recommend you hire an expert at https://experts.shopify.com.
If you want to give it a go yourself, make a backup of your theme first. Then check out this page for details on what code you would need to change: https://help.shopify.com/themes/liquid/filters/array-filters#sort
I was searching the discussion forums for this topic, and saw that this particular thread was not yet closed. Instead of making a new thread I figured I would post here.
I am looking to have items in the cart sorted first by size variant, and then by SKU number, so that when I see the order in the admin panel or on any printed invoice of orders pulled from our store, all items are sorted how I need to see them. Is this possible with the help of an expert / developer, or can this customization not be done in Shopify?
Thanks for the help.