This is going to be a very concise walkthrough because step-by-step directions are definitely not needed. Retail Clerk '90 was designed to be extremely causal.
Check the map, do all the things with the red "story event" icon. Some are triggered just by visiting the location, others are triggered by talking to an NPC in the location.
Doing the bare minimum will earn you the bad ending.
Good Ending Requirements
The good ending is triggered by doing all the optional events for at least one NPC. Completing all the events for just one NPC is enough. Again, very simple.
If you have done all the optional events for an NPC, then there will be a final one on the last day. There are three potentially available for each main character. One of these events needs to be done too. You don't have to complete all three. Just one for either main character is fine.
Perfect Ending Requirements
The perfect ending takes a little more work but is not difficult.
The requirements are:
Complete all NPC optional events for both main characters. That is six NPCs in total. Just do every optional event on the map.
Carl has a series of encounters with a strange man throughout the game. After two of them you need to follow him into a store. There will be an extra scene. Later in the game it will be extremely obvious which two stores he visited. Well, one of them is extremely obvious at the time.
Complete the trading sequence for each main character. A new step in the sequence is open each day, sometimes two steps. All the steps can be completed on the final day. No characters are repeated in or across the two trading sequences. You know you're done when Eryn has her obvious final item and Carl has a rather mysterious item.
