To start texting, you will need to add the Send SMS buttons to your page layouts.
The package comes with the buttons pre created for you on the following Standard Objects: Leads, Contacts, Cases. You can create the same buttons on Custom Objects as long as you have a Phone field available and populated.
This example will show you how to add the button for a Case.
Add Texting Buttons
Step 4: Add Send SMS Button
Click on Mobile & Lightning Actions.
Look for the Send SMS button and drag it to the Salesforce Mobile and Lighting Experience Actions.
Save your page layout.
Remember that it can take a few minutes before the button shows up on your Page Layout. Try hard refreshing (CTRL+F5) if it doesn't show up.