Code For Shopify Store- Discounts

I need help creating some code for use on our Shopify store. We want to create some code which EXCLUDES certain products from discount codes. For instance, we want to run discounts on specific collections, but some items in these collections that are tagged do NOT qualify for discount, wven if they are in this collection. Please note we are long term users of Shopify and have tried using apps and using the Script templates which do not work,