Apply a small amount of glue on a cocktail stick (middle area; whole length of the stem). Insert the cocktail stick as shown in the photo (rotary motion). If the paste is not sticking to the cocktail stick it might mean that you have applied too much glue or not enough glue.
If the mushroom is going to be inserted into the cake: leave the sticking out from the bottom cocktail stick long enough, so that the mushroom is secured properly (doesn't lean to the sides after attaching to the cake).
If the mushroom is going to be attached to the board: the sticking out from the bottom of the mushroom cocktail stick doesn't need to be very long. It needs to be long enough to secure the mushroom in the dummy properly (drying) and then it's going to be trimmed to about 6-7 mm.