How to create separate iCloud accounts with one apple ID?

In my family we have an iPad and 3 iPhones. However, all devices are using the same iCloud account so only 1 phone is fully backed up. How do we create separate iCloud accounts but keep the same apple ID?

Thanks in advance

Added (1). Is this possible in a way that we can share purchases?

No such thing, your icloud account is your apple id.

You can't do it that way, you have to create separate Apple iTunes IDs for each person. Sign out of the account, then create a new account. Sync up one of the other devices with that account, via the cloud, then repeat the process for each device in turn.

[EDIT] I just saw your added information. If you want to share purchases, then you have to share the same ID, and all the apps will be the same on all the devices. It's an all or nothing deal, I'm afraid. If the apps are free ones, you can download them individually to the devices, so you can all have the apps of your choice. If they are chargeable (cost money to download) then you can only have them on the same devices by using one Apple ID. If you share an ID, and one person deletes an app from their device, it will be deleted from all the devices. If one person buys an app, everyone will have the same app, whether they want it or not.

This also applies to music, pics and videos, where all media files will be on each device. So you will all have the same music, the same pics, the same videos, etc.