Place the meringue in a pastry bag with a No. 5 (10-mm) plain tip. To make the mushroom caps, pipe out mounds, approximately 1 inch (2.5 cm) in diameter, on a sheet pan lined with baking paper or a Silpat. Use approximately two-thirds of the meringue.
To make the stems from the remaining meringue, hold a No. 3 (6-mm) plain tip in place on the outside of the oth