Hey
What I did when same thing happened to me:
Collect rustroot and make extract.
Prospect touching the hammer in menu adventure.
Made a mine hole, about 15/20 tiles where I found the metal. As in the area where a lot of cave entrances, I thought I would find a cave under ground.
My minehole opened directly to a cave, but I thought at that moment I would need to mine to get connected to the cave system, luckily I did not need to make it.
Once I had the mine hole that worked as cave entrance, I sorrounded it with construction signs except for the lower left tile (not the corner one, but the one to the right of this most left corner sign), and placed a rune stone in the tile of this missing sign. This way no one could come up from the mine to grief my place. I take the rune stone out and replace it each time I go down and come back from the cave using my HF.
Then I explored the cave and used the little cassette drawing at Enders minimap to make a map.
When my first metal node was worn out, I prospected again the surface (it gives the data of what is right below, from surface gives level 1, from level 1 gives level 2 and so on) until I found another metal location and mined through the black void tiles to that spot. And from the cave I prospected to get metals at level 2.
Hope my experience helps you. It was a lot of advice I collected to get to be able to make this entire procces.