If you ask what is the best method of tafsir, the answer is that the best way is to explain the Qur’an through the Qur’an. For, what the Qur’an alludes to at one place is explained at the other, and what it says in brief on one occasion is elaborated upon at the other. But if this does not help you, you should turn to the Sunnah, because the Sunnah explains and elucidates the Qur’an. Imam Abu `Abd Allah Muhammad ibn Idris al-Shafi`i has said: “All that the Prophet, peace be upon him, has said is what he has derived from the Qur’an.”
Allah has said:
“We have sent down to you the book in truth that you may judge between men, as Allah guides you; so don’t be an advocate for those who betray their trust.” (al-Qur’an, 4:105)
“We have sent down to you the message that you may explain clearly to people what has been sent to them, and that they think over it.” (al-Qur’an, 16:44)
“We sent down the Book to you for the express purpose that you should make clear to them those things in which they differ, and that it should be a guide and a mercy to those who believe.” (al-Qur’an, 16:64)
This is why the Prophet(P) said: “Know that I have been given the Qur’an and something like it”
In order to understand the Qur’an, you should first look to the Qur’an itself. If that does not help, then turn to the Sunnah. The Prophet(P) sent Mu`adh(R) to Yemen and asked him: “How will you judge the cases (that come to you)?” He replied: “I will judge according to the Book of Allah”. “But if you do not get anything there, what will you do?”, the Prophet(P) asked. He said: “I will refer to the Sunnah of the Prophet(P)”. “But if you do not get it even there, what will you do?”, the Prophet(P) asked again. He replied: “I will exercise my judgment.” Hearing this the Prophet(P) patted Mu`adh(R) on the shoulder and said: “Praise be to Allah who has guided the Messenger of His Messenger to what pleases His Messenger.” This hadith has been reported in the Musnad and Sunan collections of hadith with a good isnad.
When you do not get any help from the Qur’an or the Sunnah, turn to the words of the companions. For they know the Qur’an better: they have witnessed its revelation, and passed through the situations in which it was revealed: and know it and understand it fully. This is particularly true of the scholars and leaders such as the Four Righteous Caliphs and `Abdullah ibn Mas`ud. Imam Abu Ja`afar Muhammad ibn Jair al-Tabari reports: Abu Karayb narrated to us, saying: Jabir ibn Nuh informed us that: al-A`mash informed us from Abu Duha: from Masruq that `Abdullah ibn Mas`ud said: “By the one besides whom there none having the right to be worshipped, there is no verse in the Qur’an about which I do not know in whose case and at what place was it revealed. If I were aware that anyone knew the Qur’an more than me, and I could reach him, I would certainly have gone to see him.”
Another great scholar is `Abdullah ibn `Abbas(R), the nephew of the Prophet (P) and the commentator of the Qur’an. He attained that stature in virtue of the Prophet’s prayer: “O Allah! Give him knowledge of Islam and teach him the meaning of the Qur’an.”
Al-A`mash quotes from Abu Wa`il that Ibn `Abbas(R) was appointed leader of the Hajj by `Ali(R); he delivered a sermon and read from Surah al-Baqarah, or Surah al-Nasr according to another report, and explained it in such a way that had the Romans, Turks and the Dalamites heard it, they would have embraced Islam. This is the reason why most of what Isma`il ibn `Abd al-Rahman Saddi has written in tafsir consists of the explanations of these two scholars: Ibn Mas`ud(R) and Ibn `Abbas(R).
An Introduction to the Principles of Tafsir, 1993 al-Hidaayah. Taken from Islaam.com