cyclotest 2 plus works quite simply

In the morning after waking up but before getting up, place the temperature sensor connected to the cyclotest minicomputer in your mouth, press the large round button and wait for roughly 1 minute until the unit beeps. Immediately after the beep you can read off your currently fertility status in the display. cyclotest distinguishes between 3 fertility states: non-fertile, fertile and highly fertile.

You don't have to measure every day; instead, measurements from the start of your cycle through to the non-fertile cycle phase after ovulation will suffice. Then you can take a break from measuring until the next start of menstruation.

If you want to avoid getting pregnant, you should on the fertile and highly fertile days indicated either use additional contraceptives or abstain from sex during this period.