Ok, I know this can be extremely stupid, but anyways, there's no guide, so one can avoid searching here and there.

This game, of course, is not a game - it's a mere achievement collection.

Achievements are divided into two categories: 23 of them are called "Achieve 1", Achieve 2" etc., while the other 1521 are simply dubbed "Achievement".

In order to get them all, you have to open the "game" (keep it in windowed mode, it's the same, you don't have to do anything) and click on "play".
You will soon die and numbered achievements will start to unlock.

If you see the percentages, a 91% of the players got them all, but for what concerns the first 23 achievements the percentages are different (Achieve 1 100%, Achieve 20 94%, Achieve 24 92% etc.) so, in order to be sure, let the game idle for ten minutes in order to unlock all of these so that you will have unlocked all the first 23 achievements; it won't really take more than ten minutes.

Then, shut down the game and re-open it. Without even playing it, you'll have all the other achievements unlocked on your page.