[Geoff Bacon]

It's easy to put your existing songs in a collection

Open Songs

Long Press one of the songs until checkboxes appear
Tap other songs to select them
Three vertical dots (top right)
"Create Collection from Songs"
New - Type in a New Name
It may be a good idea to not select too many songs at once (should you accidentally hit something else then you might have to reselect them).
To add further existing songs to this collection, use the "Add Songs to Collection" and select the previously used name.

When importing new songs, fill in the Collection field (it will apply to all songs in the import)

You can't delete songs from the collection screen - when removed they still exist in Songs.
Deleting from Songs gets rid of it completely.

Geoff