Download TooManyItems, open inventory, add infinite diamonds. (By the way, I am only kidding, probably strip mining is best, at about layer 13 I think).
Dig down deep to somewhere between levels 1 and 20 and dig around. And watch out for lava. You're best off creating an organized branch mine. In the process, you should find a small amount of coal, some iron, a hell lot of redstone, and some lapis lazuli. Alternatively, you can go exploring deep caves that go down to levels 20 and below.
Diamonds and Redstone spawn around the same levels. Redstone is a lot more common (in my experience). If you find Redstone, that lets you know you're at the right levels. Just dig out horizontally from there. A you need is patience and luck.
sometimes BRANCH mine
Btw: Mining down ain't helpful
"To make her love me, Whitfield was my world, today..." - Derek Pardoe
2. Find a Cave
3. Jump into lava.
4. PROFIT?????
But really, you have to get very lucky to get some of these
This.
BBCode Render failed due to reaching MaxNestingDepth(80) for Tag: spoiler