How to create a separate mobile version of a website from the desktop version without changing the desktop structure on Squarespace?

I've been trying to make a mobile user friendly structure for my website, but I haven't been successfull yet. When I preview the mobile version, everything is out of place and huge. The only way I found out I could fix it was changing the sizes (at least) of the text in the desktop version. But that doesn't really make sense because this way it will be very small for the desktop users to read everything. 

I've contacted the customer support and they have told me that I can do kind of a separate design for the mobile version withouth interfering with the desktop version by using a custom code. 

Could someone please HELP ME with this issue? How can I code the page to make it the way I want? (at least the main guidelines). 

Hi,

Can you share link to page in screenshot? We can check easier