Firstly, blob out normally, getting to decent size, and grabbing as many gold mine provinces as you can get. I took the following:
- Bambuk (Mali area)
- Bure (Mali area)
- Lobi (Kong area)
- Tafilalt (Morocco)
- Damot (Ethiopia area)
- Kaffa (Just below Ethiopia)
After you take a gold mine province, lower the autonomy to zero (centralization edict is useful), and raise production to 10 (using Encourage development edict to reduce the cost). A 10 production gold mine with zero autonomy gives around 6 gold per month. I got bored with waiting for autonomy to drop, so raised a couple of my mines to 12 production (which gives 10 gold a month, but with a higher depletion chance). Improve your mines until you have total income from gold of 30+.
After you have achieved a healthy gold income, you need to get rid of expenses until your profit per month is substantially more than 50 gold. Be prepared to sack advisors and disband your army to do this. With minimal expenses, and 30+ gold income coming in per month, you then need to reduce your non-gold income, so this 30+ gold per month represents more than 50% of your total income. I unassigned my merchants, and released vassals until my non-gold income was suitably low. Demolishing buildings might also help. Once this is done, advance to the end of the month and the achievement should complete.