As of right now, it's not hard to make money. It's just boring.
1. Massacre Missions: go to a system in War/Civil War, get Allied with one of the warring minor factions, pick Massacre Missions for them. In a Vette, you can kill 70-100 enemies in a Combat Zone every hour, and there are massacre missions paying 20+ million credits for 90+ kills. That, plus 3 million credits from the combat bonds, can reach 25-30m Cr/h.
2. Assassination Missions can pay 1-3m Cr per target, when Allied to the minor faction. Knowing how to trigger the Waypoints for the targets is a must. Can make 10m+ Cr/h.
3. Settlement Assault Missions can also pay 1-3m Cr when Allied. Pick only the + or ++ Settlements as targets, the +++ can be a pain to locate the Powergrid/ take time to complete. The + and ++ Settlements have powergrids that are easy to take down from your ship with PAs or Railguns in one shot, can make 15m+ Cr/h.
4. The Road to Riches is a beginner-friendly list of EarthLike Worlds, High Metal Content Worlds and Water Worlds, all Terraformable, meaning each scan will pay 200-700k credits; the list features 295 bubble systems with more than 500 of those planets, full list pays more than 200m Cr and can take you from zero to almost Elite Explorer rank. Only doable once (each scan).
5. The Ancient Ruins mission is given by Ram Tah as you dock at Felice Dock in Meene System (don't need to unlock Ram Tah as an engineer) and will have you scan Obelisks with your SRV in a dozen different sites for a total of 101 scans, paying 201m Cr total, doable in 5 hours, repeatable. (heard they unnerfed the previous bonus nerf)
6. Hazardous RESes and Compromised Nav Beacons give lots of credits, specially if you're pledged to Arissa-Lavigny Duval or Zachary Hudson (up to 50% bonus). Can make 15m+ Cr/h (even more, according to some legends)
7. Infamous Passenger Missions can pay ???m Cr/h (I won't say here because bad people with start shouting "nerf the exploit!" and the Devs might actually listen to them). Fit a SardineConda, fill it with peasants and haul them to a station that's really really really really far from its system center. Some time after a 40 minutes SuperCruise, you'll be filthy spacerich.
The real problem is, you won't possibly stand doing the same of those things again, again and again.
Suggestion: mix them, like Stealthie said.
Or, mix them with pure fun activities such as wing roams, Corp objectives, social stuff etc.