Once you've added the game to your library, download it and then click "PLAY".
(If it's not obvious enough, you might need to consult someone who knows.)

Once you've gone through the torment of opening up the game and pressing any button to continue, now we need to click on the right button.
(For those who are experiencing problems with images loading in this guide, the button is located under "ゲームをはじめる").

I know that you are already exhausted trying to complete this guide, but don't worry, as this is the last step that we are going to take so you can enjoy this game in its English-ish glory!
Follow the arrows in the screenshots below.
(Again, for those who are still experiencing problems with image loading, make sure that the first section on the left is either highlighted with a different color or off-centered from its original position, then go to the last setting [named "言語"], then click on the arrow next to "日本語" to change the language to English [either arrow works!]). (For keyboard-only users, highlight the last setting, then press the left or right arrow on your keyboard to change it.)

This guide has been assisted by the tools mentioned below.
MS Paint (for text on screenshots)
Snipping Tools (for screenshots and drawings)
Quillbot (for refining minor grammar mistakes)

I honestly think that the candidate developers did a great job making this demo; however, they really should try to make a way that changes language depending on where you live; this will inevitably make 90% of people who have played this game outside of Japan a tad bit satisfied.
(But you've seen this guide, so you definitely know that they didn't.)

Enjoy!