How do I set up a separate iClould storage account for an apple ID that is part of family sharing

I'm not clear on what it is you are trying to do. You can set up 5 GB free storage for each account but it is only accessible by that account. You can upgrade to have additional storage available to anybody in the family group.

Here's how to "Buy more iCloud storage". Click here--> https://support.apple.com/HT201318

Share an iCloud storage plan with your family - Apple Support - "With Family Sharing, your family can share a single 200GB or 2TB storage plan"

https://support.apple.com/HT208147