My personal choice when it comes to mining diamonds (providing I have the resources) is to go down to y=11 and create a beacon with haste along with having a diamond pick with efficiency, mending, fortune and unbreaking. Then creating a strip mine that’s a 2×1 and creating a corridor going straight in any direction.
Next once your satisfied with the length of the mine shaft make your way back to the start and count out one block to the right or left. Begin making another straight corridor in that direction heading out as far as you’d like.
Once you’re satisfied head back to the main shaft and count two block beyond your second tunnel and once again repeat this process. This can be rinsed and repeated over and over which will allow you to uncover the maximum area without mining every block out entirely.
The nice thing about this method is that once you’re happy with as far as you’ve gone in both directions or all four, you can simply begin to mine out the entire area for a huge cavern you can use to expand your base or build farms with.
Diamonds are efficient to mine with an enchanted iron pickaxe the way to find diamonds is to be near lava and bedrock the average amounts of diamonds you can find are around 3 its near a lava pool or under or not near lava at all it depends on your Y level and luck. I would suggest getting some iron at the start. Get an iron pick and a water bucket.I would make a staircase going down to y level 12 and start mining in a straight line.