Fold your napkins in thirds, then use tape to affix a stencil to the center. Soak a small foam brush in fabric paint, and dab it over the letters. Allow to dry for at least 30 minutes.
Felix Populi linen napkins (felixpopuli.com), "Love" stencil (stencilease.com), Versatex #26 ink (dharmatrading.com).
Read more at Marthastewartweddings.com: Nautical Wedding Style Ideas