Because this achievement can be very annoying and easy to miss, I decided to make a short guide only dedicated to this.
First of all, I don't know the best / fastes / easiest way to get this achievement. I can only explain what I did and what you have to pay attention to.
To be safe, I recommend to make a backup from time to time.

It can only be completed after autumn of year 2, but you need to start it in autumn of year 1.
As far as I know, you can't do it later.

Make sure that you have completetd the poisoned water mission by the 20th day of autumn of year 1.
On top of that you should already be lover with Emily and have done her questline (can't say how far you have to do it, I have done every quest. The last one was the one with the springlers).

In autumn of year 1 Emily will lose the harvest competition with her watermelon and 2 days ( not 100% sure) later, you will get a quest to find giant seeds.
After you found the seeds ( you need soap for it) and brought them to her, you will get a quest from her every monday to fertilize the pumpkin. You have to do it before 10:00am.
You have to do that every time. If you miss a week, you have to start again.

When you reach the 19th day of autumn in year 2, you should have:

• given her a heart knot
  
• raised 50,000 gols
  
• upgraded your house to the 2nd stage
  
• fertilized the pumpkin every week
  
• reached "Soulmate" with her

And don't marry her. If you do, you have to start again.

If you have done everything right, Emily will ask you the day before the festival if you'll be at the festival.
She will win the competition and right after a cutscene, you can get a quest from Albert. And at this point you need the 50,000 gols.
As far as I know, you can only accept the quest on that day.

After that, just follow the quest and in the end you'll get the achievement.

And if you have any questions, infos etc., feel free to leave a comment.