One great way is to use the EMAIL TEMPLATE feature of Field Service Project Stages.

The default Stages are NEW and DONE.  You can add others and each can use a separate email template - automatically sent when the Task/Visit moves to that Stage.

I find making a custom view provides easier access to the Stages:

<page name="settings" position="after">
  <page name="stages" string="Stages">
    <field name="type_ids"/>
  </page>  
</page>

Even without this custom view you can access them from the Projects app, via Project --> Configuration --> Stages.

With the custom view, you will now see this when you open a Field Services Project:

Click ADD A LINE to make your own additional Stages for PLANNED, SCHEDULED and LAST JOB DONE:

For each stage you can then create an Email Template that is sent to the Customer:

The DYNAMIC PLACEHOLDER GENERATOR tab helps you generate the field that will contain dynamic data, based on the Field Services Task.

CODE VIEW of this Template:

<p>To our valued Customer,</p>

<p>We wanted to let you your visit for the ${object.name} has been planned.</p>

<p>We are currently planning to arrive on ${object.planned_date_begin.strftime("%A")} between  ${object.planned_date_begin.strftime("%-H")} ${object.planned_date_begin.strftime("%p")} and ${object.planned_date_end.strftime("%-H")} ${object.planned_date_end.strftime("%p")} but this is only an approximate window</p>

<p>We will email you again the day before your visit to confirm a more convenient 1 hour window for our arrival</p>

With this Task:

This email is sent when the Task is moved to the PLANNING stage: