To fuse an item, simply hold a weapon, shield, or arrow in your hand and then pick up a material. You'll see an option to fuse.

Where to Find Materials?

Materials can be found all over Hyrule, from the surface to the Sky Islands. Common sources include: Enemies: Many enemies drop specific materials when defeated. Plants and Fungi: These are abundant in various biomes. Look for glowing plants or mushrooms.

Material Fusion — Zelda: Tears of the Kingdom Guide

Redirecting to Material Fusion — Zelda: Tears of the Kingdom Guide