Different types of plants need different amounts of light, water and temperatures. It's important to understand the requirements of your plant somewhat, if your not sure, research your plant species before deciding where to place it. Experiment with placement if the plant is not thriving - does it need to be in cooler, shady parts of your home, or with more sunlight?
Find out how often and how much water your plant needs. For example, orchids and succulents can dry out between watering, but a fern will need to stay slightly damp at all times.
To water your Kokedama, simply place it plant side up, onto a bucket or sink of room temperature water. Submerge to encourage the water to get right into the center. Soak for 10-25 minutes or until saturated, about every 1-2 weeks depending on the kind of plant and where you have it. If you keep in inside, you may want to place into a colander to drip dry for a minute before returning to it's home.
Consider how much sun and wind it's receiving as these will dry your kokedama faster. Ferns love a little mist with a spray bottle on the plant and moss every few days.
Some will benefit from liquid fertilizer added to your water... follow instructions on the packet but at half the rate recommended.
When the plant's roots are growing through the moss and the plant looks root bound you can just plant it, twine and all, into a bigger pot or the ground. Otherwise you can add more soil, moss and twine if your feeling crafty.
My Kokedama generally come in 3 different sizes...
Mini - Has about a 10cm moss ball.
Classic - Has about a 15cm moss ball.
Elite - Has about a 20cm moss ball.
These are not exact measurements, they are just to give you an idea. The Baby succulents I have made for weddings are smaller again.