Here’s the guide about how to do Ultimate Evolution on Puzzle and Dragons US:
Step 1: You need to do this in the Power Up Fusion Menu.
Step 2: Use the right materials to ultimate evolution your monster, for this example we will use a Fafnir (since that’s the available monster anyway :p)
Step 3: When you add the correct ultimate evolution materials, notice the Begin Fusion button will have a yellow color when it’s glowing (If it doesn’t have, that means the monster does not have the ultimate evolution yet).
Step 4: Tap the Begin Fusion Button and the game will proceed like the usual power up fusion (o ow..)
Step 5: Don’t worry, you will get to a new screen where you will have the choice to ultimate evolve your monster or just proceed as normal, of course we will choose the ultimate evolve.
Step 6: And there you have your ultimate evolution form of the monster.
WARNING!!: In the current US Version (4.2), these monsters can’t ultimate evolve yet:
In addition, Hades doesn’t have his dual dark ultimate evolve form (Underlord Inferno Hades) and Venus also doesn’t have her dual light ultimate evolve form (Love Deity Cosmo Venus) for the current US version (4.2).