I would do it using a stencil
but pm the creator and ask, normally people are very helpful!
take a sheet of wax paper and lay it over a print out of the letter you want, pipe with royal on the wax paper, let it dry, peel off and glue to cookie covered in fondant.
You can also put the fondant on the cookie and then lay your letter template on top of the fondant and use a toothpick and trace the edge of your printed out letter and it will leave a little of an impression in the fondant for you to go back and follow with the piping bag with royal.
HTH
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