I've just read the instructions, and the only thing you would need to do differently to dye a ball of yarn would be to omit the stirring, because wet+heat+agitation=felt! Just wear rubber gloves and gently submerge the wool so that the dye can soak into all of it, and be very gentle when you rinse for the same reason.
To get just a tiny hint of purple to take away some of the brightness, you could either use less dye than instructed, leave it in for less time, or put the yellow in after you have used the dye to dye something else as most of the pigment will have been already used.