Grey knight style lettering is usually inlaid so it\'s best making the area you want it to go into first by cutting into the surface you wantit on.
For the lettering it\'s worth making a thin snake of GS or fimo whatever your using by rolling it out. when really thin cut little strips and bend into the shape of letters.
Position the letters in thearea you want.
Use a colour shaper to flatten slightly..
when dry you can file it flatter.
It\'s a little fiddly but allows you to do finer writing