To start the mining run, you need to go to the mine first. The mineshaft is located in the middle of the map, and south of Ford Zancudo. It is also marked on the map, so you can easily find it in the legend. Upon entering the mineshaft, you will be able to find multiple places to mine your materials, the deeper you go, the more chance you have for valuable stuff. Go to an ore and stand on it, you will get a notification that you will be able to mine. Then pressin order to start mining. You can mine iron, gold and diamonds. Once your inventory is full, head to your car and press to place the goods inside of your vehicle. Once the car is filled, you're good to go to the processor. Keep in mind that diamonds do not need processing and you can sell them instantly to the shop.
Processing the minerals
Once you're finished with filling your pockets with ores, you have to go to the metal smelting, which is also marked on the map, in the south east of the city. Head inside of the factory and then up the platform. There will be a circle to process the gold, and one to process the iron (each on a different side). Pressin order to start the processing. Once done with all the processing, you're ready to go to the jewelery store to sell your processed materials.
Selling the minerals
Once the processing is done, you can go to the T.B.Jewellers shop in order to sell. The shop is located besides the Los Santos City Hall. Walk inside of the shop and speak to the person inside by pressingand sell. Every processed mineral will have a different price. Use to take the minerals from your car into your own inventory.