Not all Mega Stones require you to go through multiple steps in order to receive them, but there are always the exceptions. The exceptions come to us in the form of Mega Pidgeot and Mega Slowbro.
Pidgeot is one of those Pokémon that you almost always get early on in the game and then you just use until your fingers are bleeding or until you just move along onto another Pokémon. At least that's how Pidgeot (and Farfetch'd) always end up in my Pokémon games.
While gathering Mega Stones required a specific time of day in Pokémon X and Y, this isn't the case for Pokémon Omega Ruby and Alpha Sapphire. In order for you to grab Slowbro and Pidgeots Mega Stones, you just have to do some simple interactions.
Mega Pidgeot | Pidgeotite
Go to Rustboro City and show Mr. Stone (Steven's father) the Intriguing Stone, he will reward you with the Mega Stone for showing him. You can get the Intriguing Stone by speaking to a little girl in Verdanturf Town. She will need you help find her Shroomish in order to give you the stone, you will find her Shroomish at the middle of town.
Mega Slowbro | Slowbroite
Go to Shoal Cave, a man will request for you to collect four Shoal salts and four Shoal shells. Collect them and give it to the man asking for them, you will be rewarded with the Mega Stone.