This achievement occurs during Episode 5 - Outatime and is missable if you do not pick up the Algae Cakes.
You must firstly pick up the Algae Cakes whilst at the Expo, After you enter and leave the 'Glass House' the Hostile Guy should be on stage, now is your only chance to grab the Algae Cakes from his booth which is near the entrance next to Arthur McFly.
Now assuming you have the Algae Cakes in your inventory some time later you'll be outside an old farm house after Hill Valley disappears, now do the following:
• Interact with Cactus.
• Walk up to "Scary Mary"s door and over the tripwire.
• Interact With Tripwire.
• Dialog (Edna): Any
• Dialog (Edna): Any
• Interact with Cactus
• Interact with the Tripwire.
• Dialog (Edna): "We spent today together."
• Dialog (Edna): "I brought something for you!"
•
Use Item "Algae Cakes" to equip.
• Interact with Edna (With Algae Cakes equipped.)
Credit to T_l_M_B_0 of PST