Above all, go slow and when you feel pain, stop. This will keep these exercises safe. As for effective consider situps, toe touches, anything that moves the area.
The best exercise for the lower abs would be sit ups. Another great exercise that works the abs wonderfully is pull ups. Both of these are good choices.
Stay away from cruches, situps etc... Planks, side planks, farmers walks, and the Pallof press (I would start with holds) are all really good "core" exercises which are "easier" on the back than most...
How to loose weightthighs~~~~ 1.stand up straight 2.left either or leg 3.bend knee 4.finally twistbutt~~~~1.scwatsarms~~~~1.lift either or arm straight out in front of you 2.cross arm to other...