How to Make a Keyblade in Kingdom Hearts 1

Equip Sora, Donald, and Goofy with the Lucky Strike ability. This step is optional, but will significantly increase the drop rate of the items needed to synthesize the Ultima Weapon keyblade. "Kingdom Hearts" awards a bonus to the skill if all three characters have it equipped at the same time.

Kill Gargoyles and Wight Knights in Halloween Town until you have five Lucid Gems.

Kill Pirates, Air Pirates, and Battleships in Neverland until you have five Power Gems.

Kill Aquatanks and Screwdrivers in Atlantica until you have five Thunder Gems.

Kill Angel Stars and Invisibles at the End of the World until you have three Gales.

Play magic charades with White Mushrooms until you have three Mystery Goos. White Mushrooms spawn in every world in "Kingdom Hearts," but are fairly rare to actually find. The White Mushroom will make a gesture to indicate a spell it wants Sora to cast. The correct spell will cause the White Mushroom to give Sora an item. Each White Mushroom will request three spells with three gestures.

Go to the Accessory Shop in Transverse Town.

Use the Green Trinity ability. A ladder will descend from the ceiling.

Climb the ladder to enter the Synthesis Shop.

Talk to the Moogle in the Synthesis Shop to open the item synthesis window.

Select "Ultima Weapon" from the item synthesis weapon. The Moogle will automatically make the keyblade using the items you have gathered.