MS Word technique that is useful here:
Create new Word doc
Enter the 12x12 grid followed by some blank lines
Select all and cut/paste 12 times
You now have 12 copies of the grid to experiment with
For each copy choose a column and select the 12 rows
Choose 'Replace' from the Home menu
For the Find string ,do ([...]) where the ellipsis is the set of letters in the column (or ([!...]) if you want to find the characters not in the column).
For the replace string, choose any single character you want; perhaps just a single space character. You can even do \1 and the search will not replace any characters but just change the formatting of the matching characters! So could could turn all the characters form the column to bold red, for example.
Check 'use wildcards'
If you want, specify a format/font (e.g. bold, color, whatever)
Do Replace All (this should only affect the selected rows)
You can quickly process each column this way.