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 1: Go to Object Manager
Go to Setup.
Click on Object Manager.
Step 2: Search For The Case Object
Search for the Case object.
Click on it to bring up the Case Object configuration details.
Step 3: Open A Case Page Layout
Click on Page Layouts.
Click on a page layout to add the button.
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.
Step 5: Confirm Button Placement
Go to the Case with the right Page Layout and confirm that the button exists.