Do Lots of Four Things
It is narrated through Salman Al-Farisi that Rasulullah said ﷺ:
يَا أَيُّهَا النَّاسُ قَدْ أَظَلَّكُمْ شَهْرٌ عَظِيمٌ شَهْرٌ مُبَارَكٌ , فَاسْتَكْثِرُوا فِيهِ مِنْ أَرْبَعِ خِصَالٍ , خَصْلَتَيْنِ تُرْضُونَ بِهِمَا رَبَّكُمْ , وَخَصْلَتَيْنِ لا غِنَى بِكُم عَنْهُمَا , فَأَمَّا الْخَصْلَتَانِ اللَّتَانِ تُرْضُونَ بِهِمَا رَبَّكُمْ , فَشَهَادَةُ أَنْ لا إِلَهَ إِلا اللَّهُ وَتَسْتَغْفِرُونَهُ , وَأَمَّا اللَّتَانِ لا غِنَى بِكُمْ عَنْهُمَا , فَتَسْأَلُونَ اللَّهَ الْجَنَّةَ وَتَعُوذُونَ بِهِ مِنَ النَّارِ
O people! When the great, blessed month (Ramadhan) comes to you, do lots of four acts in it. Two acts with which you would please your Lord and two acts with which you cannot do without. As for the two which please your Lord, they are the testimony that there is no god but Allah and the seeking of His forgiveness. As for the two acts that you cannot do without, you should seek paradise from your Lord and seek protection from hell fire.
Hence, it is a practice throughout the Muslim world including in Singapore and throughout the Nusantara region that Muslims recite the following zikir throughout the days and nights of Ramadhan:
أشْهَدُ أن لا إلهَ إلا الله أسْتَغْفِرُ الله نسأَلُكَ الجنَّةَ ونَعُوذُ بِكَ مِنْ النَّار
Meaning: I bear witness that there is no god but Allah and I seek forgiveness from Allah. We ask You for paradise and seek your protection from the hell fire. 
Recitation of Laylatul-Qadr
It is also narrated that the Mother of Believers, Sayyidatina Ai’shah bint Abu Bakr As-Siddiq, had ask Rasulullah ﷺ what she should recite if she knew that it was Laylatul Qadr and Rasulullah said ﷺ:
قُولي: اللَّهُمَّ إنَّكَ عَفُوٌّ كَرِيمٌ تُحِبُّ العَفْوَ فَاعْفُ عَنِّي
Say (O A’ishah): O Allah! You are Forgiving and Generous, you love to forgive, so forgive me.
Recommended Recitation Formula
In extracting benefit from both these narrations, Muslims would recite an amalgamation of the two azkar above, in the following formula, repeatedly throughout Ramadhan:
(x3)أشْهَدُ أن لا إلهَ إلا الله أسْتَغْفِرُ الله نسأَلُكَ الجنَّةَ ونَعُوذُ بِكَ مِنْ النَّار
Transliteration: Ash-hadu an laa ilaha illaAllah, astaghfiruAllah, nas-alu-ka-al-jannata, wa na-u’zubuki mi-nan-nar (x3)
Meaning: I bear witness that there is no god but Allah and I seek forgiveness from Allah. We ask You for paradise and seek your protection from the hell fire (x3).
(x3) اللَّهُمَّ إِنَّكَ عَفُوٌّ تُحِبُّ الْعَفْوَ فَاعْفُ عَنِّا
Transliteration: Allahum-ma in-naka ‘a-fuw-wun tu-hib-bul-‘af-wa fa’ fu ‘an-naa (x3).
Meaning: O Allah! You are Forgiving, you love to forgive, so forgive me (x3). O (the most) Generous One!
May Allah forgive our sins and grant us success this coming Ramadhan!
 Sayyidi al-Habib Umar bin Hafiz recommends reciting this 3,000 times in Ramadhan. Source: Preparing for Ramadhan