Prerequisites for Critique of Pure Reason

Nothing is going to prepare you for the hot-garbage that is that is Kant's writing. He is a terrible writer. I had a Kantian professor once comment that every translation of Kant sucks because the original German sucks, to which a German national in the audience agreed.

A few years back I ran a reading group for non-philosophers who wanted to read CPR and we started with his Prolegomena to any future metaphysics. This was written by Kant to make the CPR more accessible but is still challenging; the general consensus was that it was useful and group members were grateful for the opportunity to familiarize themselves with Kant before jumping into CPR.

Finally, the quality of the translation is going to matter. Kemp Smith was the standard for a while but now days people recommend either Guyer/wood or Pluhar/Kitcher. I'd recommend trying to get a hold of both and see which is the most agreeable for you. For the Prolegomena I'd personally recommend Gary Hatfield's translation. He cleans up the language a bit (especially pronouns) and provides a list of terms that Kant uses in an unusual way.

Finally I recommend reading the translator's introduction and notes. Understanding the translator's approach to Kant, both his grammar and terminology, can provide useful insights that increase your understanding of the text.

Good luck!

Reading only Kant's own "Preface to the second edition" of the Critique of Pure Reason (CPR) is rewarding, even without knowing any other philosophical work.

Kant's preface is short. He gives a good introduction to the problem and his solution. The main part of CPR is difficult to digest. Possibly it needs several months, not to mention years :-)

In my opinion, it does not help to study Descartes, Spinoza or even Hume before. But when reading Kant one should always follow his remarks about Hume's challenge.