You should not wish to summon a nymph it is a type of water fae. The fae are free spirits would you like to be ordered to attend to someone? I wouldnt. They wont take kindly to orders, water nymphs are not all like they are portrayed in stories they can be vicious beings.May I ask what your reasons are?
I know its easier to summon certain types of nymphs depending on what your element and what you are (not in the sense of being turned, but being born as one)I have a friend who is always escorted by sea nymphs.Same as how im escorted by sylphs
a little reminder: don't say that faeries or nymphs or gnomes or salamanders do not exist if you are not familiar with ceremonial magick (actually some practicioners here do claim they exist even though they are not in to ceremonial magick because they work with them).. (i'm not against anyone here, its just there are disciplines who do believe they exists because they have summoned one-and yes, i have summoned one before- a gnome ubt just in the astral and not in the material or physical world because i'm not an adept yet)
faeries are different from mermaids.. they are of different categories.. they are called spirits (elemental) but labeling mermaids as faeries is kinda (i didn't say wrong) but kinda inappropriate thou.. its just, labeling a plastic plant to be a real plant just because it is a plant.. ;)
if you want to summon one, there are different grimoires (especially the grimoires of armadel and the grimoires of julian walde-i dunno if he is the one) who talks a bit about summoning an elemental spirit..