I think taking a more intuitive approach to start is better for me.

I started out mainly just doing crunches at first. I also try to workout everyday. I find that it structures my life better. If you miss a day then it isn’t a problem.

I started out gently too. Try for good form and do things slowly. The slower the movement the more controlled it has to be. Rushing doesn’t help. And you don’t have to work till it burns either just do a few more each day. You’ll still develop the muscles. I learned this from doing yoga.

Don't worry so much about counting out the number of reps etc. Just do what you can and if it starts to feel bad, stop that exercise for the time being and move on. Always listen to your body. Pain is not good. Be careful of your neck when you start doing crunches (don't pull on it). Don't even bother with sit ups yet.