Back to the Future: The Game - 30th Anniversary Edition

This achievement occurs during Episode 1 - It's About Time

Once inside Doc's place:

• Walk into the back room with Biff.
• Interact with Town Square Model.
• Dialog (Biff): Any
• Interact with Amplifier Controls
• Interact with Marty's Guitar
• Use Item to equip Marty's Guitar
• Interact with Amplifier Controls (With Guitar equipped)
• Interact with George.
• Dialog (George): "I can fight my own fights, Dad."
• to exit conversation.
• Interact with Marty's Guitar.

Achievement pops once you're in the back alley
Credit to T_l_M_B_0 of PST

After the opening credits you're inside Doc's house. There's a lot to look at and a lot to say but this guide will skip all of that and give you the achievement.

Walk to the back where Buff is and look at the town model. Go back to your dad George and get the guitar. Use it on the amp. Then talk George again about you handling Biff. Turn the amp up and then use the guitar on the amp again. After the scene plays out the achievement will unlock.