Commerce Customizable Products is another module that provides custom line item types like Commerce Custom Line Items, which I've shown before.

Our goal here is to sell a T-shirt with a customizable slogan on it. This could be any customization, and there could be more than one field. Here's how we do it:

1. Enable Commerce Customizable Products.
2. Create a custom line item type which will have the extra field. (We wouldn't have to do this if all products in our store had this customization.)
3. Add the field to the new line item type. In this case it's a text field called "slogan".
4. Create a special product display content type (with a product reference field) so that we can configure the display settings on the product reference field.
5. Configure the display settings to use our custom line item type (and tell it not to try to combine like items in the cart).
6. Create product and product display node to try it out.

For extra credit in the video we add the new field to the cart view, and even give the customer the option to edit it in the cart view by using the excellent Editable Fields module.