Step 1: Go to Setup 

1. Click the Gear Icon in the top right of the screen.

2. Click on Setup.

Step 2: Open Process Builder

1. Type "Process Builder" in the Setup Quick Find Box.

2. Create a New to begin building a new process

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Step 3: Name your process and define the Trigger type

1. Give your process a name

2. Give your process a description

3. Choose when your process should start. In our example, we chose "When a record changes" 

4. Click Save

Step 4: Choose Object and Specify When to Start the Process

1. Click "Add Object" 

2. Select the Object, in our example, we want to trigger the process when a Case is closed, so we chose Case

3. Choose whether the process should run only when records are Created, or when a record is Created or Edited. We chose "When a record is created or edited".

4. Click Save

Step 5: Define Criteria for this Action Group

1. Click on the first node.

2. Select your criteria for executing actions. We chose "Conditions are met".

3. Set your conditions for when actions should execute. 

4. Click Save

Step 6: Switch Back to Lightning Experience

1. Once you have completed your Bulk Send, click on Switch to Lightning Experience.

Step 4: Select and Define Action

1. Click "Add Action" in Immediate Actions Box

2. In Action Type, Select "Apex"​

3. in the next field, start typing "Send SMS", then select the Send SMS Apex Class.

4. Set the Apex Variables.

5. Click Save

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Here's how we set our variables

1. Target Record Id, Field Reference, [Case].Id

2. To Number, Field Reference, [Case].Contact.Mobile.Phone

3. SMS Message, Formula, "Hi " + [Case].Contact.FirstName + ", your case " + [Case].CaseNumber + " is now closed."

Step 5: Activate your Process

1. Click "Activate" in the top right corner