Check the tool before the ore
A wooden or stone pickaxe is not an adequate replacement for an iron pickaxe when collecting diamond ore. Bring a usable iron-or-better pick and carry resources to replace basic equipment. Do not dig straight down: the next block can open into a fall or lava.
Make the trip recoverable
Bring light, food and building blocks. Mark junctions so you can leave without guessing, and store valuable material before inventory pressure turns the return journey into a rushed decision. Diamond distribution depends on world-generation rules; an old height recommendation is not automatically correct for every version.
Separate exploration from maximum yield
Choose whether the trip is for finding a safe cave route, collecting resources or testing a version-specific mining method. The command finder can help in a copied Creative test world, but giving yourself diamonds changes the play conditions and is not a Survival mining technique.