Gnudi are marginally easier to make than potato gnocchi because you don't have to boil, peel and rice the taters. But they're still tricky, as I found.
They're a combination of ricotta, Parmesan cheese and spinach. You will find recipes with varying proportions, and egg is used to bind the mixture. There is some disagreement as to whether flour should be used in the mix itself or just to dredge the gnudi (so they won't be completely naked, scandalizing all and sundry).
Here are some tips from my experience:
1. Use fresh spinach, not frozen. I used baby spinach, cooked and then chopped very finely with my mezzaluna. Spinach should not be watery.
2. Use good quality ricotta.
3. Try to use real parmigiano-reggiano. I used aged parmigiano (stravecchio).
4. Unless you hate nutmeg (and if so, why?), by all means add freshly grated nutmeg. I recently acquired a handy bottle of it with built-in grinder from Drogheria Alimentari.
5. Use good quality butter and sage to dress (so they're not so naked after all...) the gnudi. Avoid elaborate sauces, or even a tomato sauce. I warmed the butter in a pan beforehand, then lightly sauteed the gnudi after they had risen to the top of the boiling water.
6. Avoid making this for company the first time. Experiment. I made them in small batches, varying as I went, and tasting (a lot).
7. The mixture should not be too wet. Resist the temptation to make bigger balls to save time. If the little balls flatten or have fissures, it's a bad sign.
Plenty of recipes can be found on the Web. Here is the one by the ever-active Martha Stewart, which differs somewhat from what I did (she used a food processor for the spinach and used semolina for final dusting).