On Day 1 Debbie will mention you could do a little work for Diane to earn some money, this will unlock her house as a location on your map.
WARNING: SPOILERS AND EXPLICIT CONTENT AHEAD
This game is explicit and features sexual acts and content for the entire campaign. This wiki may show that content as well as reveal spoilers for the story. Don't say we didn't warn you.
This game is explicit and features sexual acts and content for the entire campaign. This wiki may show that content as well as reveal spoilers for the story. Don't say we didn't warn you.
Part 1
Go to Diane's house which is the purple building in the top centre of your map and click on her garden area to start the questline.
She'll explain that you can't begin working as her shovel is broken, so you need to find a new one.
Go back to your house and into the garage and grab the shovel hanging on the wall. 
Once you have it return to Diane and you can get to work cleaning the garden. Remove any bugs, weeds, or vegetables that are round. You pretty much only want to leave carrots, eggplants, corn, and cucumbers. 
Go back home and sleep and return the next day to work in the garden once more. You may also need to talk to Diane to trigger the next event so do so and continue working on the garden. Eventually, when you come to work on the garden Diane will ask you to move a large wheelbarrow filled with waste. If you can't move it yet you can still work on the garden but you'll need to train your strength before you can progress the storyline any further.
Next when you return Diane will ask you to get her pump out of the shed before you start work. Go to the shed and grab the pump from the shelf on the left-hand side. 
Return to the garden and talk to Diane to tell her you have the pump. You'll ask about the contraptions in the shed and she'll tell you they're for milking. Clear her garden once more before returning the next day. Diane will ask you to drop a package off to the school but if you come on a weekend you won't be able to do it until Monday.
Grab the milk package from inside the shed and go to the school cafeteria on the second floor.
Annie will tell you she can't accept the delivery because you aren't authorised so you need to go up to the third floor to Mrs Smith's office to get her permission first. Mrs Smith will tell you to put the receipt in her draw so click on the small set of draws to the right and then on the paper inside to do so. Return to the cafeteria and Annie will accept the delivery.
When you return to Diane's house you might find she's not int he garden and you'll decide to go inside and look for her. Once in the house click on the door at the top of the stairs to find Diane's bedroom with Diane screaming while clinging to her bed pole.
She'll explain she saw a mouse and you discover that the mouse was just a shoelace but she thanks you for coming to her rescue none-the-less. Go back outside to her garden after this scene and you can inform her the milk delivery was a success. She'll offer you some milk to try as a reward and you should accept before heading home to bed.
Part 2
Continue to work Diane's garden regularly until one day she will tell you her vegetables are infested. Clear out the entire garden - notice the new bug has pincers on it's back - then go to Consum-R at the Mall to get some pesticide.
If you talk to the clerk in the store and tell her the bugs have pincers she'll recommend the spray with the green cap so purchase that one (or whichever she recommends) for $100. 
Go back to the garden and clear everything out once more and you'll get a scene showing you using the pesticide and fixing the problem. 
Tell Diane you've taken care of her bug problem and she'll give you a raise. Return the next day and once again Diane won't be in her garden. Go inside to look for her and you'll witness her masturbating in the kitchen using a cucumber. Leave and sleep before returning again.
Next time you come around Diane will be drinking and suggest you spend the day relaxing. Agree and she'll have you apply sunscreen to her back. She'll turn over to have you do her front as well but you'll get an erection and then run off embarrassed.
Come back the next day and you'll both apologise for yesterday's drama. Clean the garden and wait for another day to pass. Return and talk to Diane and suggest making a drink for her. She'll send you off to the kitchen to make one and when choosing to add a little bit more alcohol select 'yes'.
This will have Diane continuing where you left off the previous day but this time she will touch your erection making you orgasm in your pants. Afterwards, she'll bring you inside to talk about your mutual attraction and how you can both have some fun but mustn't tell anyone. Sleep and return to find her again in the kitchen. Ask her about her milking business to discover she needs a new Stainless Milk Jug and also a way to increase her milk supply.
Purchase a Stainless Milk Jug from Consum-R at the mall and borrow a book on lactation from the library by talking to the librarian about it before you return. 
Talk to Diane once more and bring up both "The milk jug" and "Milk production book" to advance the story. After the book she'll talk about the idea of becoming pregnant to increase her supply, if only she had a bull. 
You'll appear outside her house - go back inside and talk to her again choosing 'Have fun' to have sex with her on the kitchen counter. Once you're finished she'll invite you to help her with her milking, giving you access to the machine in her garage. Sleep and return to talk to her about 'Breeding' and then choose 'Me!' when she asks about a bull. She'll invite you to visit her in her shed at night for some milk production.
Sleep again and meet her in the shed the next night. Diane will ask you to collect a package from Pink, the sex shop in the Mall before you can go any further. You'll need to sleep again to make it the next day before you can collect the package. Head to Pink and talk to the clerk selecting 'Package' before heading back to the shed that night.
Diane will reveal the package was a cow costume and you can now choose the option 'Let's milk' to have sex with Diane to impregnate her.