Hi
A good way to do that is to find a WordPress theme that somewhat resembles the design you are after – not so much in looks as in layout. In other words, if your website has horizontal navigation under a header, and a sidebar column to the left of the main contents, find a theme with that layout. The widths don’t have to be exact as that can be changed.
Then it is a matter of changing the theme’s stylesheet to match your website’s color scheme, images, typography, etc. If that feels like a lot for you, your webmaster can probably help with it. If they are already creating a site from that design, its not a huge amount of extra work to add it to your blog also.
There are tutorials and articles on the web about how to adapt a theme to your design.
5 mins on Google brought me these
https://www.flisterz.com/2008/12/24/how-to-edit-wordpress-theme-or-css-for-beginners/
https://wpcrowd.com/customization/how-to-modify-a-wordpress-theme-and-to-make-it-unique/
https://hubpages.com/hub/Wordpress-Themes-How-to-Modify-the-Look-of-Your-Blog
