I used Corel PHOTO-PAINT. But you can also use any other paint program.

With my TMWFontGenerator I just saved the font as a TGA picture (with alpha channel!). Therefore I selected the graphics format '32-bit targa (TGA) with transparency'. Then I set the font transparency (tab 'Colors and effects') to 100% and the background transparency to 0 %. The result was a font bitmap with transparent letters/characters.

Then I used Corel PHOTO-PAINT to copy this TGA picture into another background picture (a photo or something else). I just used copy and paste. And because of the alpha channel the color of the letters/characters was replaced by the background picture.

best regards,
Thomas