Assassin's Creed Valhalla

There are two aspects to this achievement. The first is fully enhancing a piece of gear. Gear must be fully enhanced before it can be fully upgraded. You can only enhance your gear at the blacksmith in your settlement in England. Gear you obtain will either be regular, a bronze background, silver background, or gold background. To go from regular to bronze you'll need carbon ingots, to go from bronze to silver you'll need nickel ingots, and to go from silver to gold you'll need tungsten ingots. Ingots are marked on your map as yellow wealth dots, and turn into a stack of three gold bars when you get close. Early, low level regions of England have carbon ingots, higher level areas (100-200) have nickel ingots, and the highest level areas (200+) have tungsten ingots. The Completionist All the Way! (30G) achievement has more information on the locations of all ingots in the game.
 
Once you have the required ingots, you can go to the blacksmith in your settlement to enhance your piece of gear up to gold level. Note that if you find gear that is already gold (especially from preorder bonuses), you can skip this step of enhancing, since your gear is already fully enhanced.
 
With that done, you then need to fully upgrade your piece of gear by holding  on it in the Inventory menu. This requires a huge amount of leather and iron ore, as well as a decent amount of titanium. You'll will find a lot of these three items as you progress through the game, looting chests and finding wealth. You can also purchase these resources from various merchants in the game, provided you have enough silver. Be patient though, as you should easily collect plenty from natural progression. Once you fully upgrade your piece of gear with all 10 slots filled, this achievement will unlock.