Here is Shopify's guide to creating an alternative template for your pages:
Whenever making changes to your Theme's Code/CSS, please ensure beforehand to have a duplicate backup - should you need to revert back to your original theme after making the change.
If coding is not something you feel 100% confident with, we would strongly advise you approach a specialist in coding customisation. We can highly recommend a number of trusted specialists, all of which are grouped for your convenience under our help guide:
There is also the following Shopify Apps, which may be worth experimenting with, to see if they suit your needs and storefront: