Sorry James only just seen this.
I sold my car just before leaving France in January.
You need to print 2 copies of the form, one for the buyer and one for the seller. I completed them by hand but I believe, as Stella says, you can complete them on your computer.
Then you need to register here https://immatriculation.ants.gouv.fr/Questions-frequentes/Demandes-en-ligne/Commencer-une-declaration-de-cession
and input pretty much the same information again to register the sale. Once this has been done you will be given a code that you need to give to the purchaser so that they can register the car in their name.
Hope this helps.