Charging Insulators by Friction / Rubbing
Rubbing causes a transfer of electrons from one object to another.
If X loses electrons, X becomes positively charged.
If Y gains electrons, Y becomes negatively charged.
To help you visualize this process better, you can play with the applet below.
There are many interesting videos demonstrating charging by friction process:
Describe how the plastic becomes negatively charged and the cloth becomes positively charged after rubbing.
Electrons are transferred from cloth to plastic due to charging by friction.
Plastic gains electrons and becomes negatively charged while Cloth loses electrons and becomes positively charged.