I've recently obtained the Legendary Farmer achievement and someone here asked for tips. I ended up writing a mini-guide, so I decided to post it here so it doesn't get lost in the general chat.
First and foremost, make a checklist, or find one (like this one), or else you'll go crazy (EDIT: you'll go crazy anyway lol). More detailed tips on each group:
Produce
- Crops: Use the greenhouse if needed, plant a single seed of each, plus fruit plants and fruit trees.
- Animal products: You need both normal and large variants of items. If you're getting only larger items, try removing the auto-petter and the temperature machine, send animals out in the rain... This is especially important for the golden egg/milk, since if your chicken/cow won it's probably too happy to drop small items. I'm almost sure you don't need artisan products made from animal products, but if you do, in order to make them all you'll need 3 extra eggs, 3 milk, and only 1 of any other animal product type (from each animal AND size).
- Ocean: The underwater farm works as a Greenhouse (seasons/weather don't matter), so crops should be easy to handle. Same from "animal products" applies here for underwater animal products.