Option 1
In your calendar, either create a new meeting or edit an existing meeting. In the 'Add Conferencing' tab, select 'Hangouts Meet' (see below). Add your attendees as you normally would for a calendar invite.
Option 2
Go to meet.google.com and select 'Join or start meeting'. Click continue. Click Join Now. To add people, either copy the joining info and paste it in to an email or chat, or add people individually using the 'Add people' button.
If you have been invited by a calendar invitation, click the 'Join Hangouts Meet' link shown when you click on the invite in your calendar (see below). The same link will also appear in the email notification you got when you were invited. Once you are at the 'Ready to Join' screen, click 'Join Now' to join the meeting.
Alternatively, go to meet.google.com and select the relevant meeting to join it. Once you are at the 'Ready to Join' screen, click 'Join Now' to join the meeting
This video covers all of the steps and tips above. Please make it fullscreen if you are having trouble seeing the details.