As I recall, I used a background template I found online which is for PowerPoint (of all things). I made the text and boxes in the usual way for a PowerPoint slide, then saved/exported a PDF version for normal use, printing, etc. Today I changed that to a jpg so I could quickly blank out my birthdate on the one I posted.
If you would like an editable generic PPT file I've posted it here:
http://www.wd8das.net/RadioTelegraphCertificateGeneric.ppt
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Radio is your best entertainment value.
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Mein Ohr ganz nah am Weltempfänger...