Wa-alaykum-us-Salaam wa'Rahmatullaah wa'Barakatuh,
Indeed, to Allah we belong and to Him we shall return.
May Allah have mercy on her and make it easy for her in the stages of the hereafter and give your family patience during this difficult time Ameen.
As stated by the brother, make supplication that Allah (Subhaanahu wa ta'aala) makes it easy for your grandmother in the grave, have mercy on her and grant her Jannat-ul-Firdaws. Ask Allah (Subhaanahu wa ta'aala) by calling on Him by His Most Beautiful Names (for example, Ar-Raheem, Ar-Rahmaan, Al-Ghafoor, Al-Azeez, Al-Qawee, Al-Wahhaab), only He is the One that we should ask for help (check the link below that lists the Names of Allah and their meanings). Ask Allah the Almighty to have mercy on your grandmother at the times the Prophet
told us. For example in the last third of the night especially in this blessed month of Ramadaan, when prostrating in your Salaah, before tasleem (i.e., ending the Salaah) and after Salaah.
Ibn Mas'ood narrates I was once praying, and The Prophet
, Abu Bakr and Umar (were all present). When I sat down (in the final tashahhud), I praised Allah, then sent salaams on the Prophet
, then started praying for myself. At this, the Prophet
said:
'Ask, and you shall be given it! Ask, and you shall be given it!
(Tirmidhi, #593. There is a similar narration in Saheeh Muslim although worded slightly differently and I remember reading something similar in Bulugh-ul-Maram).
I hope this brief information helps.
Link to Allah's Most Beautiful Names:
http://www.turntoislam.com/forum/showthread.php?p=403961#post403961