There are two kinds of skywriting. Both kinds are calculated in advance, on the ground.
The "traditional" kind, one plane laying a smoke trail, requires the pilot to fly a precisely planned series of turns, i.e. fly straight on this compass heading for this amount of time, then turn at this rate to a new specific heading, fly straight for that amount of time, then turn, etc. It's all worked out and the pilot reads it from a piece of paper. Nowadays a GPS would make it easier, but skywriters are a pretty old fashioned lot, so I don't know.
The other kind is "skytyping". Here 5 airplanes fly together alongside in a straight line, and an automatic system, which ties the airplanes' smoke systems together by radio, pulses the smoke on and off as required to form the letters. Originally it was controlled by a punched paper tape in the lead aircraft; nowadays it's doubtless all electronic.
gps-computer controlled smoke/its all over from the flyby the seat of your pants
They may be able to see what the are writing.
Time (chronometer) and compass heading -
They cannot "see" what they write -