Installing Constant Cart can not be easier.
Step 1. select the "Get" button from the Shopify App Marketplace
Step 2. Approve the installation and billing charge.
Step 3. The app will automatically create the needed snippet and append the code necessary to the theme file to include the new snippet.
The app will now monitor all carts created by anonymous guests and customers that are logged in. If a customer creates a cart and doesn't checkout, the app will remember that cart and the next time the customer logs in they will presented with a message that asks them what they want to do with the cart.