Help. How do I fucking use the Megas? I have zero idea. I already finished the three basic Battle Tree challenges, sucked all the Zygarde cells/core into the Zygarde Cube, talked to Dexio and Sina many times, obtained some Mega Stones, but I can't seem to Mega Evolve my Pokémon. Help.

Also, does anyone know the complete list of Poké Finder spots? The following are the only ones I've found so far:

Melemele Island
>Hau'oli City Shopping District broken fence
>Melemele Meadow

Akala Island
>Lush Jungle
>Trailer van north of Lush Jungle

Ula'ula Island
>Ula'ula Meadow
>Thrifty Megamart

Poni Island
>Poni Meadow