After praying the Isha prayer and before going to sleep, make arrangements to wake up during the night before the Fajr prayer (for instance, you may want to set an alarm clock or have a family member agree to wake you). Though the Tahajjud can be prayed at any part of the night, if possible, it is best to pray it after midnight, especially during the last third of the night. The time two-thirds of the way between Isha and Fajr is a good time for Tahajjud. This is because Allah descends to the lowest heaven during the last third of the night, asking, "Who is asking me, so I can give to him? Who is calling upon me, so I can answer him? Who is seeking my forgiveness, so I can forgive him?'"  If you make an honest effort to wake up and perform the Tahajjud but accidentally sleep through the night, don't feel guilty. According to ahadeeth, Allah records your genuine intention to perform the Tahajjud and grants you sleep as an act of mercy. You will be rewarded as if your intention had been fulfilled. However, if you make a conscious decision to abandon your intention to pray Tahajjud without good reason, for example through laziness, then it may not be overlooked. t Wake up during the night at the time you've chosen. Once you wake, perform Wudu, the ritual ablution Muslims use to cleanse themselves before they pray or handle the Holy Quran. Traditionally, performing Wudu means using clean water to wash oneself in the following four ways:  Washing the face Washing the arms and hands up to and including the elbows Wiping the head (Masah)  Washing the feet up to the ankles Note that many Muslims (including the Prophet Muhammad [pbuh]) also choose to wash their mouth and teeth with a miswak before the Tahajjud.  If you have discharged mani earlier in the night, whether during sexual intercourse or a wet dream, then the Ghusl full-body ritual cleansing must be performed. This is done because Allah's names are pure, so, whenever possible, Muslims are encouraged to speak of Him in clean, holy places as a form of...

TL;DR in Indonesian:
Aturlah untuk bangun setelah tengah malam. Bangun dan berwudu. Jika perlu, berpindahlah ke tempat yang bersih dan tenang. Hilangkan semua urusan duniawi dari hati