Start by reading the documentation that came with the template. The documentation includes instructions on how to install and set up the template, as well as information on how to use its various features and components. https://preview.keenthemes.com/metronic8/angular/docs/quick-start
After you've set up your development environment, start customizing the template to suit your needs. Depending on the template, this may involve modifying the template files directly or using a tool like Angular CLI that incorporates the template.
If you're just starting out with Angular and want to experiment with the template, you may want to work directly on the demo to get a feel for how it works. On the other hand, if you have a specific project in mind that you want to use the template for, it may make more sense to create your own Angular project and incorporate the HTML template from HTML version into it.