Put the shaped stems in a dummy. I didn't want the bottom of the stem to be completely flat, that is why I didn't press too hard down. Leave the stems to dry. They don't need to dry out completely before attaching the cap, but they need to be firm enough, so that they don't sink underneath the weight of the cap (I left them for about 30 mins).
The larger the cap, the firmer the stem needs to be before attaching the cap. Remember that if you are using a different type of paste (not flower paste), you might need to leave the stems to dry for a bit longer (flower paste dries quicker than sugarpaste).