Use fragments, like for ToT cards or Impresario fragments -_-
There are two different systems here. If the reward is granted through the collections system, you collect fragments just by looting/buying them, and then you go into the collections UI and combine them. That's what you do for ToT cards or Impresario morphs.
And if the reward is via the achievement system, then you get a physical item that you must then use/consume, without a combination step. The Jubilee mount this year or the various fragmented collectibles from Infinite Archive work like this too.
Yes, I agree that it's confusing that there are two different systems, but at least they did spell out the usage requirement in the text of the achievement; the first three words of the achievement text are, "Acquire and use...".