Gráinne’s Gold Guides Consultancy Services speaking:
up to 50K: farm old raids for a week
up to 250K: farm old raids for a month
up to 1M: herb (might as well mine as well while you’re there) in Nazjatar 24/7 while farming old raids
1M+: git gud and sell boosts
10M: get your MBA in TSM and manipulate the AH.
Our Billing Department will be in touch about our TL;DR Service Fees.
Have A Nice Day!
(Oh, and stop spending gold. That bit is free.)
First, get yourself a starting stake. Repeating all the WoD raids on LFR, Normal, Heroic and Mythic (you can do all difficulties in the same lockout) will get you about 25K from vendoring the drops before reset. You can pick up another 15K-25K from Mists LFR + Heroic and Cata raids. ICC, Naxx and Ulduar are also worth doing.
That gets you a little gold to start investing.
Learn Herbalism and Mining and make your home in Nazjatar for hours and hours and hours. (Not for me, thanks. Other places maybe if I needed the gold; not Nazjatar! but tastes are tastes).
Repeat as needed. OK, now you have a little cash, you need to learn the art of making it make more. This involves sitting in the AH a lot, gambling by buying stuff cheap that you think you can re-sell for a profit. You will want at least the addons Auctionator and Trade Skill Master and learn to use TSM. There are videos on YT. Also read up on https://www.reddit.com/r/woweconomy/ and https://theunderminejournal.com/#eu/sylvanas and above all WATCH YOUR SERVER PRICE PATTERNS.
There won’t be any real opportunities until 8.3 in January, but keep your eye on the news of what will be in demand.