Obtain the correct Mega Stone for the Pokémon you want to Mega Evolve
Make sure the Pokémon can Mega Evolve and is holding its matching Mega Stone
Put the Mega Stone in the Pokémon’s held item slot
Enter a battle with that Pokémon
Use the Mega Evolution option during the battle if your version of Cobblemon supports it
Complete the battle while the Pokémon is Mega Evolved
Remove the Mega Stone if you want to give it to another compatible Pokémon