How do polar covalent bonds form?

Firstly, covalent bonds are formed when 2 atoms of non-metallic share their to each other to complete it's octet (eight valence electrons).

A polar covalent bond is formed when these two non-metallic elements have a significant difference in electronegativities such that the electrons are not being equally shared between the atoms, one partly pulls the electron cloud to itself making it partially negative while the other atom is partially positive.

You can learn more about this here: http://socratic.org/chemistry/covalent-bonds-and-formulas/covalent-bond-formation