You don't need an app for NFP, but you do need to make sure you understand the method thoroughly, and chart reliably, using temperature per rectum before you get out of bed of a morning. And have several months also using a barrier method, until you are sure you understand your cycle. The app is just swopping pencil and paper temp graphs for an online version.
You need to combine at least two methods - temperature and mucus, plus any other regular symptoms you know you get. And it is safer if you restrict to post-ovulatory only.
You could try full Billings method, but that advises against condoms on days you're not sure about (recommending abstinence) as you have to really know your mucus for that one.
I used to use NFP but not the app. I had only about 4 'safe' days per month without bleeding, but I have unusually long periods and was cautious about pre-ovulation days. I could probably have pushed it up to 6-7 days reasonably safely.
It depends on your cycle. My ovulation has varied from anytime between Day 11 to Day 18, so it’s red days 5 days prior to the earliest recorded ovulation I noticed. In my case this means 5 green days during period, followed by red days straight after until such time the ovulation is detected. TBH I like this approach as you are only really safe post ovulation. While ovulation can and does vary depending on lots of factors, one can’t count days and presume when in the cycle it happens. So personally I use the app as the ovulation detector and only treat days as safe past that point. For me the time between period and ovulation is too dodgy and I just can’t relax. Saying that, 10-12 green days excluding period is not bad given I am HC free, don’t struggle with my weight or moods anymore. Short of sterilisation, this is the best you gonna get while fertile (and even sterilisation has got its problems)