Xulfiqar, bro, since you have tons of experience of cars with you, Ill advise at a much higher scale, you see the reason why it isn
t recommend to use a shampoo solution is the surfactants in the shampoo can affect the clay, degrading it and massively increasing the risk of marring. Personally I've had no issues with the mix of demineralised water and car shampoo. Though over here I must advise that not all shampoos work with all types of clay-bars, you just have to find the correct dilution.
For all beginners, please stick with claying lube.....the advice given is only for the above mentioned post and for only the concerned person keeping in mind his experience.