Our customer gave us a design mockup and asked us to develop a template similar (background image, over title, text on bottom) to the mockup.
How can we develop an editable template to look like a design mockup? What are the steps/process involved to bring in the HTML and CSS to the editable template?
Go through WKND chapter 4 that covers everything you mentioned above.
 4 - Style System
Working with Editable templates and client libs are covered in this doc:
WKND Tutorial Overview
Using LESS CSS and developing components in an editable template - you can design most sites to meet your customer requirements
The PageProperties section at Creating Page Templates isn't detailed enough.
We built a sample project using GitHub - Adobe-Marketing-Cloud/aem-sites-example-custom-template-type: Sample for building an AEM Te...
and want to apply our custom CSS (global.css) to the Editable Template and then apply individual styles to components being placed in the editable template as below:
Should we apply the STYLES at Component level instead at TEMPLATE level?
You may want to check the following doc:
Creating Page Templates