If low water pressure is caused by debris in the lines how do you get it out?

If it is loose debris, simply remove the strainers at each faucet and turn on the water. If the depris is hardened and cannot be removed in this manner, replace the offending water line.

Ans 2 - The above answer will help, but not always cure the problem. Other parts where debris will almost certainly collect are any or all of the under counter shut off valves and the shower control unit. These are difficult for an amateur plumber.