Reasons for choosing any specific medium aside, if you want to pot in sphagnum using the traditional Japanese method (
which is not a kokedama, btw), you need to make sure that you don't over water no matter what method you choose to water (then again, that also goes for any medium). Many outdoor Neo growers will thoroughly soak the plant with a good spray with a hose. Some soak them in a bucket or a tub (though sharing soaking water is risky at best). Some carefully water only the moss, and others bottom water. I water mine in the shower until the moss is thoroughly soaked.
Regardless of how you water, the most important thing is to allow the moss to dry out both
inside and outside of the moss before watering again.
The moss mound should dry out to a crunchy state
on the inside, within 5 days of a heavy drench or soaking. If it takes longer than that, you need to change one or more of the following things:
Density of the moss - If the moss takes too long to dry, it could mean that the moss is packed too densely in the pot. The open core method of potting, especially using an internal netted structure makes it difficult to pot too densely, so is highly recommended. The moss should be no denser than what's needed to keep a well rooted plant stable. If the plant has few roots, support it with floral wire or stakes rather than increasing the density.
Pot porosity - unglazed clay pots wick away moisture from the moss, which help it dry faster and more evenly. Plastic pots, even with huge slits or holes don't dry anywhere near as evenly, so clay pots are highly recommended.
Air circulation - Insufficient air circulation will make the moss dry out slowly. A light breeze is recommended 24/7. Incidentally, sufficient air circulation is recommended with any growing medium. All the complaints of crown rot or disease caused by water trapped between the leaves are 100% solved by sufficient air circulation, even when the temperatures are low.
Humidity - while higher humidity is generally good for neos, the higher the humidity, the stronger the air circulation needs to be. Especially at levels above 80% humidity, ensure the air circulation is sufficient to dry out the moss completely within 5 days of a heavy drench. Ideal humidity is approximately 60%-80% at nighttime, but supplemented humidity during the daytime is generally unnecessary and may cause the plants to be prone to rot.
Once you have your potting method and environmental conditions to a point where the moss dries out in an appropriate amount of time, then you can adjust the watering frequency a bit to ensure best growth. During the growing season, you can water a smidge more frequently, once the inside of the moss is dry but not yet crunchy, and in the winter when they aren't growing, you should water a bit less frequently once the inside of the moss is fully crunchy.
If the growing area temperatures are above 60F in the winter, let the moss stay crunchy dry for just 1 to 3 days before watering. If it generally stays below 60F, don't drench, just give them a light shower, once every 7-10 days.
Even worse was then trying to figure out how to marry it up with the vase. 
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