Acquire 100 different materials.
This should come naturally while collecting the accessories and conch shells. In exploration stages, there are blue glowing lights on the ground all over and as soon as you touch them, you will pick them up. Each exploration stage has different types of materials. Once you have collecting 100 different materials, the trophy will pop. If you are struggling, note that two of the exploration stages have fishing spots that you can get their unique items Carp and Giant Catfish from. Completing Tasks from villagers in the Castle/Towns, as well as maxing friendship with officers can also gain you material items. Don't forget about the shops!