"You'll never meet one because they're imaginary and stuff"
Well, I suppose you'll never meet Jesus because he's imaginary too, or rather, in the same category of existence in which nymphs exist.
As for the original question, leaving offerings within nature and being around nature is a good place to begin. Meditate there if possible, and be receptive. If you have any clarivoyant skill, that is, if you are able to use the so-called 'inner' or 'higher' vision, or indeed any of these senses it will help you in your endeavour.
A nymph is a nature spirit with many subtypes, dryads for example are a type of nymph which reside in trees.
In my current reading, nymphs were commonly worshiped/honored in Greek folk religion. Most take the approach of honoring them like any nature spirit because rituals were not written, pretty much exactly what Hadit said. Most likely, any offerings and libations were given in sort of a relaxed, but respectful manner. (Martin Nilsson's Greek Folk Religion, Campbell's Old Stones, New Temples)
Drew Campbell, a follower and writer of Helenic Polytheism, views them as the reason why the flowers and tree grow, but not the litteral tree. Spend time in nature, give small offerings such as water, honey, or milk.
Take some time, like Hadit said, just meditating and being receptive. Like with any spirit work, do not push, and be calm. Just remember, she won't appear in a big mystic woo woo, so don't be disappointed.
Nymphs are spirits of small aspects of nature; a tree, a flowing river, breezes, flowers... I think of them as some sort of Greek (and Roman) fairies. They are really common in myths, generally involving someone falling in love with them and not being corresponded. Sometimes their stories have a good ending (Vertuno and Pomona), or a tragic one (Sirinx and Pan).
As they represent the spirits within the elements of nature, I'd recommend you to meditate in quiet and lively place, somewhere a nymph would live. A park or somewhere deep in the woods if you have the chance. As you meditate, try to feel their presence, like you would with any other spirit. Connecting with an element may help. I wish you luck and a blessed day.
^Hadit, you were the first one to bring up Jesus here, as a reply for somebody else saying that nymphs are fantasy creatures.
1. Jesus being real or fake, doesn't make nymphs neither real nor fake. It has nothing to do.
2. You can believe that Jesus existed or not, that even if there was a Jesus, maybe he wasn't what he or others claimed he was. But it still has nothing to do with nymphs. This is another case, we're talking about a person who may have existed if we take the Bible only from a historical point of view.
3. At any rate, I believe that the Jesus we're being sold by Christian religions (or other religious theories) is as mythical as the tooth faery. So what?
You can believe that Jesus was real and nymphs aren't, you can believe that both are myths, or you can believe in both, if you like.