See http://www.univ-tlse3.fr/inscriptions/?RH=rub02 (sorry only in French). The best is to ask a French speaking colleague to help you, but otherwise, here is a summary : 
For M1 and M2 Students, not coming from University Toulouse 3: 
- the dates to register are from 06 July to 20 July and from 22 August to 14 September
- the registration is mandatory online.
- you will need the acceptance letter, the document proving you have at least a Bachelor Degree, a copy of your passport, a picture. (see here the documents: http://www.univ-tlse3.fr/medias/fichier/liste-pj-pour-iaweb-2019_1561706470432-pdf?ID_FICHE=328422&INLINE=FALSE)
- first you need to pay a common fee for all French students here: http://cvec.etudiant.gouv.fr/. The is the Contribution to the Student and Campus Life (translated...). It is 91€. 
- then, you will need to downlad the registration files at the University Toulouse 3. For this, you must first register  on https://iaprimo.univ-tlse3.fr/iaprimo/. You need you identification number (either eCandidat or CampusFrance). At this stage you will have to pay your registration. The price for registration is for  the registration itself and for the mandatory health insurance (217€ if you are under 28, but there are plenty of special cases, see http://www.univ-tlse3.fr/medias/fichier/info-secu-mutuelle-et-tableau-couleur_1498830392508-pdf?INLINE=FALSE). All this in done online. 
- then you need to create your intranet login at the University : https://univ-tlse3.fr/comptenumeriqueut3
- when you have done all this and your registration accepted by the administrative staff, you will be able to (through the intranet) : download your registration certificate, take an appointement to have your student card.


- be aware that the courses start on September 23th, 2019, for M1 and M2.