Trying to learn how to fully use Serum Powder...

[[Serum Powder]]

So, I understand that I can exile my hand and draw a new hand instead of mulligan with Serum Powder.

I also understand that if I already have mulliganed I need to put back the necessary amount of cards before using Serum Powder. (ex. If I have mulliganed twice then I would first put back two cards and then if I still have Serum Powder then I can potentially exile my hand and draw five cards.)

What I'm trying to learn about this interesting card is this...

When exactly do I have to announce that I'm using Serum Powder?

Do I choose to mulligan and then pass to my opponent to see if they mulligan and then choose to use Serum Powder based on this information or would I have to do it BEFORE knowing if they are going to mulligan?

I'm thinking I do this IMMEDIATELY after establishing what my hand is (meaning if I haven't mulliganed yet then my hand is what's there but if have mulliganed then again I need to put back the necessary amount of cards and then immediately choose to use it) but I'm just trying to make sure I'm going to do it right.

Interesting and fun card. Thanks everyone for the help! 🙂