How to create a user function to count for a particular characters in a cell

Is there some reason why you couldn't use the formula solution provided in that previous question?

If you want to know how many instances of the letter "o" occur in A1:

=LEN(A1)-LEN(SUBSTITUTE(A1,"o",""))

If you want to know how often the letter "o" occurs in the range A1:A3, use this array formula:

{=SUM(LEN(A1:A3)-LEN(SUBSTITUTE(A1:A3,"o","")))}

To enter an array formula, DO NOT type the curly braces, and use Ctrl+Shift+Enter instead of Enter to finish the formula.

I want the function be available in all the spreadsheets without re-entering the function each time.

Even if this were deployed as a UDF, you would still have to write a formula using that UDF in all of your workbooks.

Going the UDF route only makes sense if the functionality you need is not readily available through the built-in functions.

Patrick is right here.
Even the link you provided offered a inbuilt formula based solution which you can use in any of your workbooks.
But if for any reason you want to use a UDF, you may place the following code on a Standard Module of a blank workbook and save it as an Excel Add-in (.xlam) and then enable that Add-in from the Developer Tab --> Add-Ins.

Function CountCharacters(cell As Range, Character As String) As Long
   CountCharacters = Len(cell.Value) - Len(Replace(cell.Value, Character, ""))
End Function

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Then you can use the above function on the worksheet as below....

=CountCharacters(A1,"character to count")

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I have gone through the steps several times, but I must be doing something wrong.  While it shows up in the Add-Ins list, it doesn't show up when I start typing the function name.

"But if for any reason you want to use a UDF, you may place the following code on a Standard Module of a blank workbook and save it as an Excel Add-in (.xlam) and then enable that Add-in from the Developer Tab --> Add-Ins."

Both the answer and video were very good. I was not familiar with Inserting a Module.

You're welcome Alex! Glad to help.