MB3 190a: Laws of Writing on Shabbos (340:4 - part 2)

The איסור דאורייתא only applies when you write on a permanent surface with a permanent writing medium.

Writing on one's skin is אסור מדאורייתא.

Cutting and etching out the shapes of letters.

Going over with a pen letters that are already written.

Writing only one letter on Shabbos, writing only one letter but one that's significant.

Writing two letters on two different surfaces.