First of all this is not some scheme, its simple manipulation of processes put in place by some websites.
The first step is to find a company that you enjoy, Boxed.com, lootcrate, blueapron etc.
the second step is to dig into there subscriptions. Find websites that offer a referral code or eve a referral link.
Referral codes are by far easier to manipulate.
The third step is to post the code or referral link to any and all coupon sites, a quick search of "submit a coupon" will pull up endless lists of sites.
the fourth step is rinse and repeat, not all codes take off very well and not always on the first try either. be patient and proactive. but if done correctly you could feed your house for free or even get a subscription box for free
I hope this helps out, also feel free to check my link at the bottom for a step by step guide for boxed .com