Salam to you, too! :D
It can be done anytime between Isya’ and Fajr, as long as you’ve slept (short nap would suffice) after Isya’. The recommended time would be the last-third of the night.
Niyyah: Usolli Sunnatat Tahajjudi Rak’ataini, lillahi ta’ala. (I pray the sunnah tahajjud prayer, 2 rakaats for the sake of Allah)
Visit this page for the step-by-step tutorial: http://www.wikihow.com/Perform-the-Tahajjud-Prayer
Wallahua’lam.:)