The photo explains it all. You make ice 'cubes', like you'd normally would make 'm.
But fill the shapes only half with water and the berries, wait till it's frozen and then fill the whole thing up.
This provides the berries from floating. You can actally use any shape ice ice tray. Then you just need to add a paperclip, half sticking out, to hang it with. Ofcourse the decorations only work outside and it must freeze. When they start melting, it's a treat for the birds!
Have fun and Merry Christmas!
P.S. Thanks goes to Judith Bähner, it was her idea. I just posted the instructable.