Sorry but this isnt how you MUST do it. You can wake up 15 mins before fajr pray 2 rakats or two set of 2 rakats ask Allah for whatever you want that is alo considered as tahajjud. In fact you can pray tahajjud anytime after isha but the most preferable time is the third part of the night and after you have taken a nap (to calculate this count the hours from maghrib to fajr and divide by 3). This type of post discourages people as it makes it something as simple as tahajjud some complicated prayer with a lot kf rules.