1 - Start by visiting a supermarket or grocery store.
2 - Check the products to see the ones that have some contests or prizes.
3 - Most promos nowadays require an installation of an app on your phone or visit a site to submit a code.
4 - Discreetly start scanning the codes or qr codes.
5 - Accumulate points and cash out in the store or online.

Note: In most promos you can exchange points for real items. If you don't want the item, just list on classifieds sites with a below the market price.