Multigrid method
 

I plan to apply multigrid method ,specially FAS, in my code for NS equation and I have problem in doing that.
in fact, I don't know how to find L (ap & an) (L*U=F) in coarser (in restriction steps)or finer (in prolongation steps) grid? it means if I should calculate it by interpolation or should do by calculating it by new grid?
and if second is true, how should be obtained, because we have mass flux in each face and for do it, I need to obtain face velocity.
thanks in advace