Find a cave, find rustroots, make extract. That's the easy part.
Now extract works like that: the better the ql, the larger the area you prospect. Make a jar (best ql you can get) with high ql water and rustroot, so you'll have high ql extract. With this extract, you go inside cave and check in wich direction the tunnel goes. Outside and underground maps go in the same direction, so if your tunnel goes, say, north, you can follow his course outside by going north.
Now get outside, follow the tunnel direction you found, and prospect every, say, 20-40 tiles, depends on your extract strength. If you see a red circle on the ground, you've found metal veins around. Since the last world tho, "veins" may very well be a couple tiles, even just 2-3, that's why you should use a good ql extract for start prospecting, so you'll easier find even traces of ore.
To better locate the veins, make a jar with low power extract: this will reduce the radius of research, making easier to locate with precision your vein. If you followed the tunnel direction, this way of prospecting will also make it easier to mine toward the vein, and you'll have to mine less to get to your precious ore
