1. Go to this page and follow the instructions for installation : http://blog.metaclassofnil.com/?page_id=582
After installing and opening it...
2. Click User Whitelist, write "Stardew Valley" and save.
3. Click Edit Settings, press the '+' button on the bottom, write "Stardew Valley" and Create
4. In the newly opened page, you can write your settings that are specific for Stardew Valley and will overwrite other global settings.
I didn't touch the global settings and only put these lines in the new profile :

#scalingType lanczos
scalingType nearest


~~~MOUSE FIX (some users have problems with the mouse being locked into a specific area, add these two lines to the profile you've created just in case):

modifySetCursorPos true
modifyGetCursorPos true

save and close it.

(Optional) If you want to take full resolution screenshots, the program can do it as well(doesn't work well with really high res), but first you need to edit the keybindings.
Click Edit Keybindings, click the 'U' button on the bottom and click Yes to create a new profile.
Here you can set your keybindings, mine are the following:

takeFullScreenshot VK_SUBTRACT
takeScreenshot VK_MULTIPLY
takeHUDlessScreenshot VK_DIVIDE

These correspond to the numpad minus, multiply and division keys.

5. Make sure the program says "Activated" on the left side of the window.
6. Open the game, load your save, open the settings and switch to Fullscreen Mode.
7. Set your desired resolution from the menu
8. After choosing the resolution, switch back to Borderless Windowed mode and it should keep the resolution + allow you to alt-tab without any issues.
9. Take large screenshots of farm and post in thread


*NOTE- I have not tried this with the various injectors and trainers and whatnot, so can't confirm it works. 
The main thing you need to watch out for is the name of the executable file and set the Whitelist and Settings profile name according to that.
The default executable name is "Stardew Valley.exe", so you put "Stardew Valley" , without the .exe extension.

If you're getting missing .dll errors, make sure you followed the instructions in the first link, you need all the redistributables mentioned there. Other than that I can't help you out that much with issues.