Mine ores, hunt animals for leather and collect ingredients then invest a few perk points in Smithing and Enchating and you're on your way to swim in septims. Seriously. I wanted to level up the three manfucaturing skills to get some over powered equipment and potions at the early stages. Sell the excess, invest the money into better materials, rinse and repeat.

I have to admit, I spent the first five levels scrapping by until I spent enough time with gathering/hunting but after that it was paradise on Tamriel. It did help that I roleplayed as a solitary Argonian hunter, favoring the bow over chatting with the townsfolk at daytime. Not to mention sleeping at cleared out dungeons at nights instead of comfy tavern beds. (My speech skill is still only around 40 at lvl 87 :<)

Created potions with a high alchemy skill sells for a ton, enchant iron daggers with banish, pickpocket magical rings and amulets. And if you dont want to use any of these methods you can always just level up until the enemies drop ebony gear on a regular basis.

By actually playing the game...?

Could be he's spamming it on every spelltome he finds LOL
Enchanting's a good way to earn money, especially the paralyze and stamina damage enchantments.

Smithing and Enchanting. A Legendary Daedric Warhammer with Banish and Absorb Health enchantments is worth over 13,000 gold in my game. The Banish enchant I got from a bow I purchased for 5,000 gold in my early 30s and the Absorb Health came from something I found in a dungeon. Bada bing, bada boom. I'm richer than the emperor with nothing to spend money on. So now I don't even bother selling them. I just make them and stuff them into a sack in the corner.