Stick Top + Bottom Header with Marketing Banner (West Template)

Site URL: https://www.annamasonlondon.co.uk/

Hello! I am trying to create a sticky header for this website where I can still keep the marketing banner. Here is an example of what I would like to create - https://shoprodarte.com/

Does anyone know how to create this code without the weird space that would be created on top as you scroll? 

Thank you in advance!