Abs workout without sit ups or gym?

Hey
What is a good abs workout without doing sit ups (they cause to much pain to my back). And i can’t afford gym membership. I have seen and tried ones where you only go a little way up and hold it, they don’t damage my back as much.

Also does anything that causes a burn in your abs mean that its working?

So what are your tips? (link to video demonstartion if possible thanks.)