Weak form of Naveir Stokes equations in COMSOL.

If a physical model is not listed in the wizard, we can use Comsol's weak form to enter PDE's (governing equations of a system) in their weak form. How can it be done ?

for example: 2 equations of momentum (2D flow) coupled with continuity equation

ρ v.∇v=−∇p+μ∇2v

∇.v=0

This model (well-known) is for calculating velocity and pressure fields. How to make weak form and then enter to comsol for solution?. Your help will be strongly appriciated.