To add a section to the service, you will want to be on the job information page of the service you would like to add the section to, click on the '+' button next to any service, scroll down to the Section drop down and select your section of choice (you can only have one section per service.) This will drop them into the Section, allowing you to exit the Service Details pop up, and click on the orange Save button in the upper right hand corner.
Note: sections are generally used to group services together for display on the proposal, and should function like this by default. If you see that this is not happening when you try to use the feature this may have been removed from your template during customization. Please contact our support team to have this added back for you.