The downside of course is that once you start hitting a satellite with a signal they know exactly where you are and the operators will call the cops on you. Your signal is not only potentially interfering with someone who paid for the transponder, but it also impacts their energy budget.
I've been told that if you rent a transponder it's important to put a signal up on it pretty quickly, otherwise pirates will attempt to "borrow" it from you, and they can be hard to evict.
There is a lot to cover, from calculating link budgets, scramble codes (related to anti-jam), waveforms, pointing a dish, calculating frequencies, reading channel allocation docs, tradeoffs of different frequency bands, etc... However, the basics are pretty basic. You modulate a series of bits onto a carrier at a specific frequency and radiate it at a specific point in the sky. The transponder on the satellite amplifies the signal, shifts it a fixed number of Mhz, and retransmits it back towards the Earth.
There are some satellites that act like flying routers and actually decode the packet back into bits before re-encoding it for transport, but generally the added complexity has made this approach unpopular. One constellation that does this is Iridium, because it then forwards packets between satellites until it can be downlinked to the ground.
[1] https://en.wikipedia.org/wiki/SIGSALY [2] https://en.wikipedia.org/wiki/T-carrier