How do I file an exemption in Missouri if a creditor has a hold garnishment on my checking account?

From what I can tell you "simply" write a letter offering your defense, copy to person holding Judgment, Court (which may or not be involved, depending on which Court it is), Attorney for credit (if there is one), clearly stating your reason for an exemption, providing proof if available.

how do i file an exemption in missouri if a creditor has a hold garnishment on my checking account?

What is your grounds for the exemption?

You submit it to the court that issued the garnishment, and copy the creditor and/or their attorney, you should site in it, the reason, including the code for it.