From the Admin Dashboard, click on Students on the left hand side. Click any student to open their Student profile. The profile opens to the Overview tab. In the Personal Information section, you'll find the basic details that can be entered by clicking into the field.
Click "Add custom field" in order to add additional details to the student profile. The Add custom field window will open where you'll select the Custom field type, either Short text or Email. Once selected, enter in the Custom field label/name. You'll see the custom field added to the student's details with a fillable field.
Once a custom field is added to one student, it is visible on every student's profile.