On the Campaigns page, you can either edit the campaign (pencil icon) and add a new step or create a new campaign. I will explain how to add a new step to an existing campaign.
To add a new step to an existing campaign
Step 1: Click on Edit (pencil icon) for the campaign. The Automated Messages page for that campaign will open up.
Step 2: Click on Add new. The New Automated Message box will pop up.
Step 3: Complete the form, making sure to choose SMS text for the message type, and click Save. The Automated Messages page will refresh to show your new step.
Step 4: Open the new step by clicking on its down arrow to the right of the Trash Can.
Step 5: Complete the information in the form and click Save. Note: This step is where you choose which user is going to get the ping. Pro tip: you can use merge fields in the message to fill in the subscriber's details to send to your user.
Now, each time a person opts in to that particular campaign, the user you chose in Step 5 will get a message.