How do you people make so much TBT?

black market sales, clearly

Sell whatever you have or spam the forums.

I just recently came back here, but what I did was post a lot on the forums, sell ACNL items, pokemon and do art commissions. I have to admit that art commissions were the best way for me to get them, though. I'd often get 500-1800 tbt (depending on what the commissioner wanted me to draw). But even if you aren't artistic in that aspect, selling in game items can all add up pretty fast. If I recall, I sold a bunch of hybrid flowers, bushes, saplings and minerals and made quite a few bells from doing that.
Honestly you just need a good strategy and the right mindset. ^^
Good luck saving up!