Requirements: access to the nether, diamond pickaxe, 16 ancient debris, 16 gold ingots, one set of diamond armour, furnace with fuel, crafting table, smithing table.
The first step for this trophy is to acquire 16 ancient debris blocks, which you can only find in the nether. I found that level 18 in the nether is for ancient debris what level 12 is for diamonds in the normal world. In the nether, if you carefully dig down to level 18 from your starting position and then just mine forward (so digging yourself a one square wide two square high tunnel) you will soon find ancient debris. I did my mining in the nether waste biome but I'm assuming the biome type doesn't affect the spawn rate. The blocks are brown and have a lighter brown/cream pattern that looks a bit like a target, with squares instead of circles. You'll know you've found one when you try to mine it because it's like obsidian in that it takes longer to break and must be mined with a diamond pickaxe to be able to collect it. If you find one block, dig on the same level around it and above and below too as often there will be one or two more close by. While searching, make sure not to stand up against the next blocks you're breaking because you will find lava at some point - I found a two square gap gave enough time for me to quickly place a block to stop the lava flow before it reached me - I just diverted around the lava problem, and then kept on with tunneling onward. Also, while you need a diamond pickaxe to mine the ancient debris, iron pickaxes are fast enough for mining netherack so make sure you have a few on hand or take a crafting table, sticks and iron ingots with you. You might be digging for a while and it's a shame to waste a diamond pickaxe on netherack. This also ensures you don't mine forward too fast so you can watch out for sudden drops on the ground in front of you. You'll be at level 18 so a drop probably won't kill you but better safe than sorry, right?
Once you have 16 ancient debris you can return to the normal world and smelt them in a furnace (or blast furnace). You will then have 16 netherite scraps. You'll need your 16 gold ingots at this point, because the next step is to make netherite ingots; 4 netherite scraps and 4 gold ingots will make one netherite ingot. Use your crafting table to make 4 netherite ingots.
The last step is to upgrade your diamond armour with the netherite ingots. Using your smithing table, place a piece of diamond armour in the first slot and add one netherite ingot in the second slot - the result will be a piece of netherite armour. Do this for all four armour pieces.
Once you have all four pieces upgraded, don your new attire for a dark but stylish look ... and a new trophy for your collection.
Note that this trophy can be glitchy, as it was for me. If it doesn't pop for you, save your game and exit Minecraft completely. Restart and go back into your world and hopefully the trophy will pop - mine took a few seconds, so be patient.