The scholars discuss this issue in detail in the "Principles of Jurisprudence". We can say here that when the Quran and Sunnah does not directly provide a solution, the jurists still refer to the verses of the Holy Book and the narrations of the Prophet and the Imams (peace be upon them) to get the general guidance in such situations in order to arrive at the practical duty in them.
Some narrations can be used to reveal basic principles denoting the practical duty of the individual in certain cases, like "everything is permissible unless one knows that it is prohibited".

The role of the Legislator is for the Prophet (peace be upon him and his holy progeny) only. Lady Fatima (peace be upon her) is infallible as the twelve Imams are, and they were all conveying the Islamic teachings from the Holy Prophet. However, what has been narrated specifically from Lady Fatima Al-Zahra (peace be upon her) is very little.

The innovation in religion is deeming something to be a part of religion when it really is not a part of religion. For example, the Taraweeh prayers are an innovation, since the supererogatory prayer is not legislated to be offered in congregation. Offering it in such a way is considered as innovation in religion.

‘Irfan’ is a word derived from ‘Ma’rifah’, which is the personal cognizance and understanding that affects the self. It has become a term for certain ideas related to the universe and existence in general, and to God, His attributes and His names. On the other hand, ‘irfan’ has the objective of the purification of one's self, and may entail claims of discoveries and observations beyond the usual capabilities and perceptions of humans.
Some might consider this knowledge to be similar to divine revelation, so it may cause one to fall into dangers, as one may then not be able to distinguish between truth and falsehood. Indeed, because of this, many have fallen into clear violations of the Sharia. This has been one of the reasons of misguidance, until it reached the level that religious obligations were dropped and the mention of God in the heart was considered to be sufficient without the need to perform the religious obligations.
Hence, it is necessary for the believers to beware of getting into this field without discernment. Instead, they should adhere to the teachings of the Qur'an and Sunnah, which emphasize acting upon the Sharia duties which are related to the relationship with God, like prayer, fasting, pilgrimage, zakat, etc., as well as those obligations related to social duties, such as being dutiful to parents, having good relationships with relatives, having good communication and empathy with other believers, giving sufficient attention to family members, with due regard to their rights and raising them in a proper way, and other obligations that are explained in books of religious jurisprudence.
The Islamic sources have emphasized the performance of religious acts in a particular prescribed way, so the believer can fulfil his religious responsibilities. They also recommend various voluntary acts of worship to allow us to get closer to God and to lead to our good deeds gaining divine acceptance.
The Islamic sources have also emphasized non-reliance on this world and to not be overly concerned about it, and they have explained that the world is a temporary abode and we are on a short journey through it. They say that one should equip himself in this finite world with good deeds and high morals, and only this can be an asset to take to the next world and ensure one’s eternal happiness, amongst the righteous followers and the Prophets and Imams (peace be upon them).
The Islamic sources have also given emphasis on the importance of exhortation and remembrance of the Hereafter, and reflection on the demise of the world and its manifestations. One should not become negligent or forgetful of this. Thus, the values and ideals are enriched in himself and his soul will no longer pay attention to the world and its delusions that lead to sins and wrongdoings.
Believers are advised to give due attention to the recitation of the Quran and the supplications narrated from the Prophet and his household (peace be upon them all), as well as their advice and teachings. All of these have a great impact in raising the soul and purifying it from vices; and before all of this, believers are urged to rely on Almighty Allah and seek His mercy and grace, which will lead to their guidance and protection from sins and evil.
Almighty Allah said: “And whosoever places his trust in Allah, He will suffice him.” He also said: “As for those who strive in Us, We surely guide them to Our paths, and surely Allah is with the good-doers."

It is not right to depend on other than the holy Quran and the reliable narrations of the Prophet and the Ahlulbait (peace be upon them) in the matters of religion, including Irfan. There are many such narrations that explain the recommended worship acts and the kind of relation a man should have with Almighty Allah. Muslims are encouraged to gain the religious and godly knowledge from these original sources only.

Almighty Allah made the religious laws of worship, like praying and fasting, and other laws like the obligation of wearing hijab and the prohibition of consuming wine, general to all mankind. The Muslim that does not obey Almighty Allah in observing one or more of those laws can be punished for this disobedience. Non-Muslims will be punished because of their refusal to believe in Almighty Allah and His Messenger (Peace be upon him and his progeny). If the non-Muslim can present an acceptable justification for his refusal to believe, then this justification will also apply even more so for his disobedience of Allah Almighty with respect to the religious laws of worship. If he cannot present a justification before Almighty Allah for his refusal to believe, he would have a lesser chance to do so for not observing the rulings of the Sharia.
It is the responsibility of the infallible Imam to implement and apply the Islamic laws on non-Muslims.

There is no doubt that Almighty Allah does not accept any religion but Islam. Almighty Allah said: ”And whoso seeks as religion other than Islam it will not be accepted from him, and he will be a loser in the Hereafter". (3:35)

From some narrations reported from the Imams (peace be upon them), we can understand that they used the term “fareedha” to refer to the obligations mentioned by Almighty Allah in the Holy Book, and the term "wajib" refers to all obligations, whether they are established in the Quran or from the teachings of the Prophet and the Imams (peace be upon them).

Islam is the belief in Almighty Allah, the Prophethood of Mohammed ibn Abdillah (peace be upon him and his holy progeny), and in the Hereafter. Whoever submits to these beliefs is considered a Muslim, and the rulings pertaining to Muslims apply to him.
Iman is believing in the mentioned three tenets of Islam, plus the divinely ordained authority (wilayah) of the twelve Imams (peace be upon them).

Fiqh is the study and deriving of the religious rulings from the religious sources of proof: the Holy Book and the Sunnah of the Prophet and the Imams (peace be upon them). These rulings were known during the beginning of Islam by direct referral to the sources, as the Prophet and the Imams (peace be upon them) used to explain them to people in simple and understandable ways. As time passed, and people became farther away from the era of the legislation, views differed in understanding the religious texts and the conclusions they derived from them. Fiqh changed from being simple and understandable by the lay people to a specific field of knowledge that required experience to determine the religious rulings. From such a development, the Principles of Fiqh (Usool al-Fiqh) was developed and become a separate field. It discusses the general principles that the jurist requires in the process of derivation.
In short, Usool Al-Fiqh is the study of the general rules that the process of derivation depends upon.