Sit ups to flatten my stomach?

I see people on here saying that they did sit-ups to flatten their stomach because it only focuses on the stomach. In that case,any are there specific sit-ups I should do,how many days a week,and how many times a day.

Perfect-ably ,can you send me a link of a picture/vid so I can see how to do it.

Btw,I have nowhere to jog/walk/run/or swim.
So please suggest something that doesn't take me leaving the house.