What you needPolystyrene ball
Gold headed pin
Start by threading the sequins onto the pin then inserting the marshmallow onto the pin and then pushing the needle into the poly ball.
Keep doing this until the whole ball is covered in marshmallows, pins and sequins. When you are finished glue on a loop of ribbon for hanging.
You can add this to the tree as is if but if you are worried about ants, spray the ball with spray varnish to coat the marshmallows with a protective layer.