If there is a shift you want to give away, find it on the schedule and click on 'Give away' below the shift. (Mobile app: go to 'Give Away a Shift'.)
You'll then select who you are offering the shift to. When you're ready, click 'Continue' and your shift will be added to the Shift Pool for others to bid on.
IMPORTANT: The shift is yours until another Employee bids on it and you receive a notification that your Manager approved someone else to take the shift.
Next up: How to take extra shifts.