If you are attaching the mushrooms onto a cake, leave the sticking out cocktail sticks long. If you are attaching them to the board trim them to about 6-7 mm. I stuck a piece of sugarpaste (mixed with CMC; balls of paste flattened to about 3-5 mm) to the board. I applied glue onto that piece.
Next I covered it with a thin layer of sugarpaste (plain; no CMC) and textured it with a dresden tool (small circular moves; some to the left, some to the right).
Then I attached the mushrooms - apply some glue on the bottom of the stem, hold by sides of the stem, press down all the way to the board (through the sugarpaste which is covering the board). If the cocktail stick is too long - trim it.