CPQ uses HTML tags for each of their content sections. And it is very different than other document creation tools that work with Salesforce. Many admins find it difficult at first to use. Here is some simple advice:
Use a WYSIWIG Online Editor
There are a bunch of editors online and I will try to update this post with some choices. But the secret is that any HTML content needs to be simplified for CPQ to use it well.
I find using an online editor versus using one with all the bells and whistles creates a better result.
Limit the Tags
Inspect your HTML code and be certain that it is as simple as possible. Many editors insert lots of tags especially when it comes to formatting, like the <span> tags, that CPQ at times just doesn’t show it perfectly. So be careful with using tags.
I also found that random extra lines are inserted when extra tags are used. Honestly try to use as few of the HTML tags as possible.