First, soft reset your game until you start the game with Dialga's unique cry. This sound is the same sound that plays once you begin to battle Dialga in space-time distortions, or even when starting the game with a shiny Legendary when your save is corrupted.
1. Before going into the Spear Pillar and activating the Red Chain, save the game. Make sure to save in a new save file, if possible.
2. Go into the Spear Pillar and activate the Red Chain. Go up to Dialga and interact with it to start the battle.
3. If Dialga is not shiny, reset the game and try again from step 1.
It may take a significant number of tries before encountering a Shiny Dialga. Remember, encountering Shiny Pokémon is a matter of luck, and there is no guarantee that you will encounter one even after many attempts.