On your Google Calendar, drag to create a new event for the times you want to make available for meetings. Set the title of the to "#free" (no quotes). You can also include other text like "#free for office hours" or "Hold for office hours #free" as long as "#free" (no quotes) appears somewhere in the event title.
Step 2 (optional).
If you are sharing your Google Calendar availability with other people who are not using FreeBusy, you might also want to set the event to "Show as Free". If the event is set to "Show as Busy" they will see you unavailable for that block of time.
Step 3 (optional).
Configure repeat settings if you'd like this explicit #free time block to repeat on certain days of the week or days of the month.
Save the event. You're done!