Note that "jako" is not a declension of the word "jak", it's a different word, and neither of them are the same as the word "jaki".
If I understand correctly, there are basically three ones that DON'T decline:
jako - "as" (On pracuje jako model - He works as a model)
jak - "how" (Jak to możliwe!? - How is this possible?)
jak - "like/as" (Jestem głodny jak wilk! - I am as hungry as a wolf!)
And then a fourth one, the only one which DOES decline:
jaki - what kind of (Jaki smak? - What flavor?)