The first way is branch mining. Start by digging a four block tall staircase down.

Once your at Y = 12, start digging straight. To know your Y level, press F3 and look at the second number on line XYZ. If your play on Minecraft PE, go to the top of bedrock and count 8 blocks up.

You will find a lot of ore after some mining.

Once the branch gets long, make another one off the side of it.
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The second way to mine is caving. Just walk around caves and mine any ore. Try to stay around Y = 12 but it's ok to be on a different level.
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There are monsters in the unlit areas and it's easy to get lost. Bring a good sword and some armor! A good trick is to put your torches on the right so you always know where you went if you die.
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My last and favorite way to mine is speed mining. This only works in Minecraft Java edition. It is the most expensive because you need a full beacon. Start by digging out a 9*5*9 area at Y = 12.
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You need 2 stacks and 36 iron blocks. Start by filling the 9*9 area with iron blocks.
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Pyramid the iron blocks until you get a 3*3 and place the beacon on top. Make sure the beacon has no solid blocks above it.
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Set the beacon to haste II and you need a diamond pickaxe with efficiency V.

The creator of this guide has not included tools