I think the easiest way is to use PTC's. PTC's are websites, that pay you for viewing adverts. This means that the owner of this site gets a certain amount of money for showing you this advertisement. When you register and view this adverts, you get a piece of that cake.
At most sites you’ll get paid 0.01$ when you view that advert. If you find other people that could also join this PTC, you invite them by using your ref link and make them your referrals. For every one of them that joins as your referral, you’ll get also a certain amount for every ad they view.
Now for example, if a PTC gets 2.5$ to advertise an ad to 100 people, than you would get 0.01$ for your view and 0.05$ if one of your referrals views it. So 100 people need to view this advert and they have to pay at most 0.015$ for one viewer (for his click and his referrer). The PTC gets 0.01$ for one viewer. So if they charge like 2.5$ for every 100 viewers that PTC is quite sustainable and you can be quiet sure they wont turn into scam.
PTC sites charging less money for advertising than they are paying to their members are unsustainable and cannot live forever. Unsustainable sites are scams. So if you want to join a legit PTC, you have to check how much they charge for showing ads and how much they pay for viewing them.
