HOAS is the main student housing foundation in the capital area. Apply as early as possible — demand spikes in August.
Create an account on hoas.fi and fill in the application with your personal details and study information.
Upload your certificate of student status (from Sisu once you have study rights) when requested.
Wait for an apartment offer. When you receive one, accept it by paying the deposit and uploading the requested documents.
If you reject a first offer, it's unlikely you'll get another in time for August — start looking at alternatives (UniHome, BOTH, AnttiKorppi, or private) as a backup.