There are two types of buttons that can be added to worksheets: ActiveX controls and Form Controls.
Right-click on the button and do the following:
1. If the button is an ActiveX control, you'll see an option "View Code".
Click that and it will open up the VB Editor inside a procedure like this....
Code:
Private Sub CommandButton1_Click()
End Sub
Just insert a call to your function
Code:
Private Sub CommandButton1_Click()
Call Header
End Sub
2. If the button is a Form Control, you'll see an option "Assign macro...".
Click that and it will open an "Assign Macro" dialog window with a place to enter the macro's name.
There is a list of available macros below, but those are Subs, not Functions.
You can type your function's name in the box and if it is found by Excel it will be linked to that button.
Unless you need to return values from your Procedure, you could make that a Sub instead of a Function.