Ok gonna be a long messege
The items you need for the tailor dude you get from chests in the areas around the map, you can get it from silver or gold chests, in the bottom right you can see how many there are in each area, use the item finder (the tresure chest item) to find them easily
To use the item finder just equip it, then in the bottom next to your exp gain thing you gonna see a chest icon, everytime you win a battle or find a item it increases making your odds to get chests better, when you find a silver or gold chest it resets, you can fuse it to make it faster for you to find items
the stone tablet give it to the fisherman on the beach, then he gonna give you fish, give that to the cat
To fuse items you need minerals, like gold and silver, use them on your items that give you buffs ( like more arcane, speed, cooldown,etc.), but to make them fuse you first need to aquire the skill, when you use a item (lets say arcane +5) that buff starts to become avalible on your skill points page and only when its maxed the the game allows you to upgrade the item (just so you wont lose the buff), the tresure chest item is different you can upgrade it rightaway since is not a skill you can learn, the items go in this order: silver -> gold -> orichalcun -> and the last one i forgot the name but its a triangle item, there are also items to upgrade your money bag so keep that in mind, and the only item that needs the silver upgrade is the chest so there is only 1 silver ore for you to get (its in the cave where you need the light to go, which you also use fuse to get since you fuse the lamp and the light stick)
About the item Phineas gave you i dont remember what was it, gonna need you to remind you what was it so i can answer