You can try the torch / water-spray method. This will require:
- Propane or other gas torch or perhaps a lighter.
- Foam letters
- spray water bottle
- paint (light and dark)
- foam paintbrush
- foam roller
Spray the foam with water, then carefully run the flame over the foam until it warps being careful to keep the flame moving. The water will prevent regions from deforming so the letters will deform nonuniformly. After flaming each letter, paint with your light color with the foam brush and get in all the cracks and crevices. Once done with the brush, use the roller with the dark color and lightly paint the surface of the letters to only paint the high points of the letter surface features. If you do dark color first as shown in the video, the letters are harder to see and blend with the black arch.