FTM and contraction counting

So I am having contractions, thats was established with my NST. During my NST they didn’t hurt, it was literally just my belly tightening. I don’t know what an actual contractions feels like. But I’ve been having pains in my lower belly and back since I’ve gotten home from the hospital. How do you track contractions? My birthing class taught me but which apps are best and how do I know when to start counting or something?