Hi Domm, first you warm up throughly by jogging a mile or so. Then you do your intervals, if you know what pace you normally run at, try to run 20 to 30 secs per mile faster, there is not need to sprint as fast as you can. Each 400 interval is best done in about the same time. After each interval don't stop immediately but slowly jog or walk to recover before doing the next one. Once you have done your intervals cool down by jogging slowly and then walking.