First, you'll need to create a HubSpot account, enroll in the Education Partner Program, and submit your HUB ID here.
Once you do this, your students will receive invitations to join your portal. There is no need for your students to do anything but check their inbox, open up the invitation, and follow the instructions to create their account. They will then be added as users to your account and you'll be able to oversee all that they do.
Please keep in mind that you will also need to edit their permissions, so that they have free rein in the software. Guidance on how to do that is here.