You can use it whenever they seem blocked - it sort of sucks the mucus out its not drops. You just press it in, pop the end in their nostril, press the other nostril close then release and it gently sucks the mucus out into the nozzle.
I have one of the snot suckers which you put one end up your baby's nose and the other you suck (don't worry there is a filter) incredibly effective especially if my husband does it. Everyone i know with the bulb sort say they are hopeless compared to the ones you suck. Also put saline spray up their nose first as that softens it and makes it easier to come out
Put some Karvol in an oil burner in her bedroom (obviously well away from her cot or moses basket) about 2 hours before bedtime and that will really help unblock her little nose while shes sleeping. If you havent got an oil burner putting a few drops in a bowl of hot water in the room has the same effect. Hope you little one is better soon.