Customize Master Page

AnaUser My recommendation would be to go back to the customer and work through mockups that are supported in the modern environment. 

However, if you are hard set on implementing something like this in the mock up, the docs.microsoft.com site has all the resources you need. Be prepared, it's A LOT of reading. Also note, that not all the information is going to be on this page, but you'll to click through the links in the Terms and Concepts section, to get a full grasp of what you are trying to do.
https://docs.microsoft.com/en-us/sharepoint/dev/solution-guidance/sharepoint-site-branding-and-page-customization-solutions