To upload a receipt, sign in to the app and then tap the bright red "CLAIM" button which is located at the bottom of your screen. You'll be guided through the upload process from here. You can either select the items you wish to claim from a list, or you can scan the product barcode(s).
You can redeem multiple offers with a single receipt. However, each receipt may only be uploaded in claiming an individual offer a single time across the CheckoutSmart network which includes CheckoutSmart, cashback sites, brand sites and other partners.