Depends on what your body is used to and your current fitness levels. I think there is way too much emphesis on heart rate when doing cardio for fat loss these days. The best way is to see what works best for you. The three main cardio options are intervals (sprint/jog), steady state cardio (medium paced jogging) and low intensity cardio (fast walking). Intervals burn by far the most calories and keep your heart elevated for longer after the completion of exersice. I would recommend, after a 5 min warm up - sprint 30 seconds, jog 1 minute. Do this for 20-25 mins and you will burn a massive amount of calories.
Do your cardio after your weights sessions, your heart is already elevated and you will ultimatly burn alot more calories this way. It is also the best, if not the only way, to spot reduce fat. If you work hard on your legs and the perform cardio, the amount of nutrient dense blood in those muscles will help oxidate fat loss in that area....this is true with other bodyparts too. Does not make a massive difference, but it all helps.