I used Fertility Friend and I paid for an annual subscription. You need a basal body thermometer which is accurate to two decimal places, I bought mine of Amazon for £5 ish.
When you temp, you should be aiming to temp around the same time each day (including weekends!), so if you're up at 6.15am in the week, then be prepared to set an alarm on weekends to do temp at that time! Your thermometer needs to go under the tongue and roughly in the same place each day, mouth should be fully closed. You must take your temperature before you even move out of bed.
It's not always necessary to temp during your period as your temps are not as reliable, though you can do if you want. Once you've put your details in, your app will likely register you've ovulated as you will have had three temps above your cover line (average temp), as your temperature increases in luteal phase due to the rise in progesterone.
You may wish to continue temping until your period or bfp, sometimes I did, sometimes I didn't but you can find yourself obsessing if you do! I temped for around 15 cycles and I found it very accurate.
This is my chart from when I fell pregnant. Hope this all helps!