You need to fuse kinstones with mysterious walls that look like this:
Please note that you cannot obtain the Mirror Shield until AFTER YOU HAVE CLEARED THE GAME. You must defeat Vaati first. If you have not done so and still ache to have the Mirror Shield, you are out of luck.
The Goron Quest has to do with just one Goron. If you have not seen this creature trying to dig in Lon Lon Ranch at some time during your game playing, you may want to take a look. The Goron, if you have not seen him, is located on the very southern tip of Lon Lon Ranch, to the west.
First, you need to fuse with the farmer Eenie. He is located in the Eastern Hills, just under Lon Lon Ranch and where the Goron is digging. One you fuse with him, the first Goron will become magically strong and break through the first door in the cave. Now you know one of your goals: Help the Goron dig through the cave.
From there, you must fuse with the five Mysterious Walls. For each Mysterious Wall you fuse with, a new Goron will appear in the cave at Lon Lon Ranch and help the original Goron dig. Their locations are as follows:
It will be right near you if you have just finished fusing with Eenie! Break open the cave with your Mole Mitts, and fuse with the wall in the cave. The Mysterious Walls as you will find often are hiding behind some stone, so you'll have to dig them out.
You will need the Roc's Cape for this. Stand at the entrance to the lake, and jump from island to island, going north (you may find a heart piece along the wya if you have not retreived it), until you reach the strange door that you can dig open.
Located just near Syrup's Hut. You will find a strange looking door just near the stairs that lead to the path to her house. Dig through and find the wall inside. Fuse to make another Goron come!
Located on the far west side of the Highlands across a bridge that leads north. This bridge is a bit easy to miss, as it is in an intersection between the bridge west to Crenel.
On Crenel Wall, next to the Hermit's house. If you haven't found the Hermit's house, go to Crenel Wall. Climb the wall completely and exit, then walk left and re-enter the wall. Continue climbing down until you reach the Hermit's house, upon which you will see a hard to miss strange door.
Once all the Gorons have come to aid the original Goron (making six Gorons total), you will be able to fuse with the sixth goron, all the way to the rightmost side. This will wake up Biggoron, who lives up on Veil Springs. There is no possible way you can miss Biggoron, because he is so huge! If you go and talk to him (After beating Vaati, of course!) he will ask to eat your shield.
Give him your shield. Now, you have to be extrememly patient and willing to be Shieldless for quite a while. Warp around to a few places, spend some time killing enemies, and then go back to Biggoron. He will give you back your Sheild. Don't be discoouraged at its mutation, though, this is exactly what you want... your old Shield is now the Mirror Shield!