How to use via-points

Hi,
I want mpc_local_planner to follow plan a path through my custom via points but i am confused how i can do it i set `objective/type` to "minimum_time_via_points" and also set `global_plan_viapoint_sep` to 0 but now i don't know-
1.  how should i publish via points ? 
2. at what frequency i should publish them ?
3.  and when to publish them ?