First things first. To begin, make sure you have at least 400K, or else please go back to the previous section. Use that 400K to buy 4000 Cowhides at the Grand Exchange for about 100 GP ea.
Here is a brief video that goes over the method:
Now with that being said, slap on some sun screen because we're heading to the desert city of Al Kharid!
To enter Al Kharid you need to have 15gp with you, which you will pay as the toll on the front gate which is located East of Lumbridge Castle. Once you have passed the gate, head south until you come upon a small desert city that has a bank.
What profits could be made in a hot and dry city like this? Well, read on and prepare to drop your jaw at the next profits table.
Tanning That Hide
Since you already purchased your 4K Cowhides, the process to convert the hides to your first million is on its way. To start off, head to the Al Kharid bank and withdraw a few thousand GP and 27 Cowhides.
Head North of the bank to the tanner.
Then use that 680k to buy 6K cowhides. Repeat the process and...