Use 1 to 2 tablespoons (15-30 mL). To make the most foam, add it to about 2 inches of bath water, and then, kneeling from outside the tub, with fingers spread and palms horizontal, use rapid opposing long arm motion to splash the water very hard. It's like trying to clap very loud, but with your thumbs instead of your palms facing each other. When the foam stops rising, rapidly fill the bathtub with the rest of the water.
There's no danger of runaway foaming from this material in a tub with built-in jets, if it is the type whose air intake is inside the tub, i.e. facing the bather. the heavy wet foam will rise only as high as the air intake and then be recycled. The following instructions are for the Vita Bath; adjust as needed for your model.
Use 1 to 2 teaspoons (5-10 mL). Aim jets down. Run water at least high enough to cover water intake; this may be done with the bather in or out of the water. (If the controls are remote, of course you'll need to turn on the jets before entering, or have someone else operate them while you're in.) The air intake (the little screw cap on the inside wall of the bathtub on the same side as the water intake) whould be opened as fully as possible without the cap's being completely removed. (The cap protects the works from hair.)
To get rid of the suds, if the water has at least some hardness, close the air intake completely, direct the jets upward (if the water is deep enough that it won't be thrown out of the tub), and run the jets while dissolving some bar soap in the water (if you haven't used soap already). When the foam has noticeably diminished, stop the machine.
I refer to the type sometimes resembling outboard motors that clip on to the edge of the tub. There's no danger of runaway foaming. Open air intake fully. When water is high enough and device is running with bather seated, pour 1 tablespoon into water just ahead of the intake.