Want to offer 60 or 90 min sessions to clients and give myself 30 mins buffer between appointments

Hi!

I offer sessions that are either 60 mins or 90 mins in length. I want to offer this choice to my clients (I do not offer 30 min sessions). How do I go about doing this? Also - I want to leave myself at least 30 mins between appointments. Again - how is this achieved?

Thanks!

Ann