Instructions
Right-click on a crafting bench and create a shovel, an iron or diamond pick, and several torches.
Use your shovel to dig straight down into the surface of the world, switching to the pick when you encounter any material that is not dirt. Diamond blocks can only be found between the first and 19th layers of the map, so continue digging straight down until you hit bedrock, a dark grey material that will not break no matter how long you mine it. You are now on the first layer of the map.
Dig horizontally with your pick, laying down torches as you go for vision, until you come across a rock with blue flecks in it.
Right-click on the rock with blue flecks in it with an iron or diamond pick until the rock shatters. This produces a diamond gem that will then be transferred into your inventory as you walk near it.
Continue digging and laying torches until you have gathered all of the diamonds you need to complete your project.