After you installed the app, you don't have to do anything else. Only need to search product and enable the pre-order option on it, the PreOrder Me button will shows up for it.
Product A has Red, Green and Blue variants
You can customize pre-order for each variants with difference options.
Also, you can disable pre-order for certain items in anytime.
In the shopping cart page, the PreOrder-Me will identify which is the pre-order items and appends a label on it automatically(you can customize the label style and text as you want).