Okay. First you need to start off by getting around 100k to start this.
Once you have this go to the G.E. and buy a Ring of Dueling 8 and some Varrock teleport tabs.
After you have these buy as many black dragon hides that you can. (Might take awhile to buy them for mid price but just leave it in til you get them)
Then use your ring of dueling to go to the Al Kharid duel arena.
Then proceed to the bank to get your hides and go to Ellis and tan them for 20 gp each.
Proceed to do so until all your hides are tanned then use your tele tab to go to varrock and go to G.E. and sell the tanned hides.
(The tanned hides have always sold immediately for me)
They sell a little over mid price but if you keep doing this you will start making a little money.
Enjoy and tell me what you think.