To begin a session, click on your contact's name. This will open your contact area.
[for an explanation of how to add a contact, see the post 'How do I invite my contacts?']

In the contacts area, click on the either the 'audio' or 'video' call buttons to start your call.

Await a call response from your contact - when your contact answers, the session will start.

For information on what you can do in a session see the post
'What can I do in a Sylo session?'