Hi Ivana,
At Gare de Lyon you will find
yellow TGV ticket validation machines (click for a photo). On the front is a small grey slot where you can insert one end of the ticket (length wise). Use the left end of the ticket, where there is usually a bar code and a SNCF logo. Insert the ticket face up (magnetic stripe down).
I've updated
the article on using TGV ticket validation machines with more specific instructions on how to use them.
Thanks for your question.
Ben