Register your Finnish address with the Digital and Population Data Services Agency to get a personal identity code (henkilötunnus).
Prepare your documents: passport or EU ID card; residence permit (non-EU/EEA) or EU registration document; your full Finnish address with apartment number; a certificate of student status from Sisu (My Profile → Study rights).
Book an appointment at dvv.fi OR attend the university's Check-In event during orientation week (much faster).
Fill in the electronic form on dvv.fi before your appointment. Citizens of Nordic / EU countries use the EU form; others use the non-EU form.
Attend your appointment in person. Bring all documents.
You will receive your personal identity code by email or post within up to seven weeks. Then send the last four digits to International Student Services from your university email (not full code by normal email).