Care to elaborate? When, what, how often, etc...
Just do 4-5 exercises in a row with no rest,and repeat one more time,right after you train.
Preferably on the days where your workouts are the shortest timewise and/or on easier days.
Diet and cardio are a must to really see them once they are developed.
Example:{Crunches
{Lying Leg Raise
{Roman-Chair Situps
{Rope-Crunches
Do these twice (two cycles).............the second cycle will be a bit harder and you`ll probably get less reps..........don`t rest until all 4 exercises are done,then wait 2 minutes,and repeat.