Well, there are a few options.
You can either build one from scratch using html and javascript. You can do so, as you would do with a “normal” website but you have to integrate a few functions to let caspar control the template. Unfortunatley the HTML-Producer Wiki-site on github is still missing, however someone did a blogpost about that with the most relevant infos. https://blog.evilgeniustech.com/casparcg-html-producer-basics/
The other way is to use a WYSIWYG-editor for building templates, as many of us are not web designers. Many people use Adobe Edge Animate, even tough its no longer developed. There is a libary (e2c) to integrate it with casparCG. You can find some info on that here on the forum: WYSIWYG software to create HTML Templates