Michelle, generally you have to add new fields to your e-mail template for every checkbox. Start with adding "New item" to your form, then choose Checkbox. Then you will see a button for adding new checkboxes. In every checkbox's settings (gear icon) you will have two fields: Checkbox Label - where you have to describe your option, and Field Name - where you have to add a UNIQUE name for every checkbox ("You must write field name with hyphen(-) with lowercase. No space, UPPERCASE, Capitalize is not allowed. This name should match with Form template value. Never keep empty this name").
Example of field naming:
Checkbox Label: Option no. 1 > Field name: checkbox1
Checkbox Label: Option no. 2 > Field name: checkbox2
Checkbox Label: Option no. 3 > Field name: checkbox3
Checkbox Label: Option no. 4 > Field name: checkbox4
Checkbox Label: Option no. 5 > Field name: checkbox5
Finally, you have to make/update your email template by adding string for every checkbox with that UNIQUE name, placed between double brace like {{unique-name}}.
Example of template:
<p>Options selected by the client:</p>
<p>{{checkbox1}}</p>
<p>{{checkbox2}}</p>
<p>{{checkbox3}}</p>
<p>{{checkbox4}}</p>
<p>{{checkbox5}}</p>
As a result, only the selected checkboxes' labels will be sent in the message. So if the client only chose options 1, 3 and 5, the message will only contain those labels.
Example of email:
Options selected by the client:
Option no. 1
Option no. 3
Option no. 5