If you use a watering can, you might find that the head won't clog as bad from the medium if you mix the material in a bucket and then strain out the solids when you pour it into the watering can. I usually wash the medium two or three times to be sure of getting out the stragglers, and then I might put the wed medium on one or more fire and mound(s) and then wash through it into the mound(s).
I'd use lukewarm water, but don't use very warm water. I'm a bit suspicious (maybe unjustifiably) of using chlorinated city water, so I might try filtered water, spring water, or well water if you have it. I've seen references to using distilled or deionized water, but I'm not too sure about what sort of osmotic conditions that would create when mixed with the medium. I don't know what sort of electrolytes are in the typical medium, and I wonder about how well the nematodes would tolerate a hypo-osmotic environment. It seems likely that the dilution water is even more important for the product that comes on sponges instead of in medium.
As for application times, I presonally believe they're best applied in the evening aroung dusk. Ultraviolet light can affect them adversely, as it can many microorganisms, and infrared has the potential to dessicate those that remain exposed on plant material. Either can cause mortality. So, put them out in the evening and water them in right away after you've applied them. I've had some that demanded food and wine, but they settled for some lightly marinated flea larvae with chocolate-covered fire ant grubs for dessert.
