It is simple to select a study on the WordGo app, providing that you are a group guide.  

First you navigate to the 'Group Details' icon which is the second from the left at the bottom of your screen.  You then select 'Course', scroll all the way to the bottom and 'View course library.'  

You can browse the available studies and by clicking on one you open up a description.  Once you have found the course you want you can click on 'Start Course.'

You will have the option to change meeting details for your group at this point, or you can simply select 'Confirm' and either skip, or send notifications to your group members about the new course you will be studying.

If you have any further assistance please raise a ticket here to contact a WordGo customer support agent.

Here's quick video walking you through you how to do this: