Each person who can should make 100 Nawaful rakats recite Surah Ikhlas 1000 altogether in the hundred rakats. (i.e., 10 times in each rakat)[1]

On the 15th of Sha’ban, The Holy Last Messenger (Sallallahu alaihi wa sallam) was seen in sajda for a long time and he was heard reciting this Dua in with tears:

All talibeen are requested to learn this Dua by heart and say it in sajda as many times as possible and weep for forgiveness for themselves, their family, community, Islam and Muslim ummah at large.

This will be lengthy and tiring therefore after finishing the 100 rakats freshen up and then make four special rakats for this Dua remaining in sajda for as long as possible.

You also have the option to offer 8 rakats and in each rakat recite after Surah Fatiha, Surah Qadr once, and Surah Ikhlas 25 times.
And of course everyone will fast on the 15th of Sha'baan.

Endnote

1. Hazrat Ghaus-el-A’zam relates that Sultan Hasan al-Basri (upon them and their descendents be the blessings of Allah) said: “Thirty Companions of Sayyidina Rasulullah (sallallahu alaihi wa aalihi wa sallam) related to me that Allah Ta’ala will look seventy times upon one who performs this prayer on this night, and with each glance He will fulfill seventy of that person’s needs, the least of them being forgiveness.”