Spot exercising your abs is best done with good ol' decline situps.. Weighted situps help too.

However, if you're looking to lose fat then the best way to lose your waist and slim your bum/thighs is simply by losing weight.

Tummy, bum, thighs.. These are where your body naturally stores fat. When you lose weight, this is where the weight drops off from first.

So, you want to slim your waist and thighs... You're going to need to do some cardio, and follow a proper diet... And you'll see, that's where the weight will disappear from.