Yakuza 3 Remastered

This achievement is obtainable once you meet Kamiyama in Okinawa (part of the story, you'll be given an extendable staff from him) and he lets you know about his weapon modding business right above the Ryudo family HQ. There's also another Kamiyama location on Park Street in Kamurocho.

To Mod a weapon you need two materials along with the base item as well as the weapon recipe, along with a service fee depending on the weapon. Weapon materials can be found by defeating enemies, bought in various places and found within coin lockers. Recipes can be found through some sidequests as well as coin lockers, but are easily bought from either Kamiyama or BEAM in Kamurocho. The recipes themselves are in the Valuables section of your inventory and will tell you what you need to craft them, just make sure to go through the menus carefully, sometimes you'll have to select a weapon and scroll through that weapon's recipes to get to the one you want. This is easier the farther in the game you are, as you'll have access to more materials and recipes.

The easiest weapon to craft (at least for me) was a Nunchuck with two coat hangers as materials bought from the M Store found either in Hatsumachi or in Ryukyu Boulevard, however the Yinglong Tonfa/Chinese Dragon Tonfa comes in handy for a Legend playthrough.