Question
How do you:
(a) Invoke peace and blessings on the Prophets (a.s.)
(b) Seek forgiveness for all the believers and Muslims who have passed away?
Answer
There are numerous ways to invoke peace and blessings on the Prophets (a.s.), one of them is to recite the following:
اللهم صل على محمد عبدك و رسولك النبي الامي و على آل محمد، اللهم صل على محمد و على آل محمد صلاة تكون لك رضاً و له جزاء و لحقه أداء، و أعطه الوسيلة و الفضيلة و المقام المحمود الذي وعدته و اجزه عنا ما هو أهله و اجزه أفضل ما جازيت نبياً عن قومه و رسولاً عن أمته و صل على جميع اخوانه من النبيين والصالحين يا أرحم الراحمين
Regarding this, I can recommend an excellent Kitab written by Hadrat Shaykh Muhammad Zakariyyah which would give you an insight into the different blessings one may invoke on the Prophets (a.s.), along with their significance. It is entitled, “ Fada’il al-Durud”.
The answer to the second part of your question follows a similar thread: there is no one way of seeking forgiveness for the Muslims who have passed away. One way is to recite the following ayah of the Qur’an:
ربنا اغفر لنا ولإخواننا الذين سبقونا بالإيمان ولا تجعل في قلوبنا غلا للذين آمنوا ربن إنك رؤوف رحيم
Or you could read the following ayah of the Qur’an رب إغفر و ارحم و أنت خير الراحمين whilst making the intention that may Allah forgive, and have mercy on all the Muslims who have passed away and all the Muslims that will come till the Day of Resurrection, as stated by Shaykh Yunus of Saharanpur who is one of the most eminent Hadith scholars of our age.
I can also recommend Hadhrat Shaykh Muhammad Zakariyyah’s, “Fad’il al-A’mal” which has a chapter on istighfar. This book not only informs one of the different types of istighfar, it also imbues one with the relevance and significance of istighfar.