This one is going to be short, so don’t blink:

The simplistic way to generate 20k a month revenue online is finding and maintaining 500 people, who will pay $40 a month in a recurring membership created by you.

So create a membership site, deliver $500 a month in value to those members, and charge $40 a month in fees per member.  Yes, some people will leave each month, so go get enough people to maintain the 500 member level.

No excuses why you can’t (i.e. can’t get traffic, can’t retain members, can’t, can’t, can’t, etc…) … just do it.