I've made a number of decals in the past, without much problem. Remember of course, that unless you have an 'ALPS' printer, you can't print white. Also, on clear decal sheet, light colours lose a lot of density once transferred onto the model, so overlaying two or more of the same decal sometimes works. Even black can look a touch 'thin' sometimes, depending on the background colour it's applied to.
For German crosses, I've scanned existing decals, re-sized the scans as required, and then printed them onto white decal paper, trimming close around the design before use.
I've used an existing font for such items as Werke Nr, although I can't remember which font it was, without trolling through the lot!
For Luftwaffe code letters, I've used commercially available sheets, possibly more common in 1/48th scale, although again I've also scanned decals or art work, if the colours needed have been dark, such as black, red, blue, where some have needed a two-decal overlay. When this hasn't worked very well, I've painted over the decal, or just hand-painted the characters.
Remember, when printed, to allow the sheet top dry for at least 15 minutes, and then spray a good quality acrylic varnish over the design, to seal it. Note that 'Future' is NOT suitable as a decal sealer on the printed sheet.