To begin with, one must have a general idea of the desired subject area of research.
Visit the web-sites of the 8 research laboratories affiliated to the Doctoral School to have an idea of who does what.
Contact by e-mail the faculty members which interest you, or by default the head of the desired research group.
Send an email to the ED Infomaths secretariat with your CV, your Master's grades, a letter of motivation specifying the desired research topic.