Basic principles of Tasawwuf (Sufism) according to Imam Al-Nawawi

Shakyh Muhammad Amin Kurdi defines Tasawwuf (Sufism) as a knowledge through which one knows the states of the human soul, praiseworthy or blameworthy, how to purify it from the blameworthy and ennoble it by acquiring the praiseworthy, and to journey and to proceed to Allah Most High, fleeing onto Him.

Its fruits are the heart's development, knowledge of God through direct experience and ecstasy, salvation in the next world, triumph through gaining Allah's pleasure, the attainment of eternal happiness, and illuminating and purifying the heart so that noble matters disclose themselves, extraordinary states are revealed, and one perceives what the insight of others is blind to.

Imam Al-Nawawi defines Tasawwuf (Sufism) as follows

The rules of the Sufi way

The basic rules of the way of Tasawwuf (Sufism) are five

1. Having godfearingness privately and publicly
2. Living according to the sunnah in word and deed
3. Indifferences to whether others accept or reject one
4. Satisfaction with Allah Most High in scarcity and plenty
5. Turning to Allah in happiness or affliction

The attainment of these basic rules are five

1. Godfearingness is attained by scrupulousness and uprightness
2. Following the sunnah is attained through caution and good character
3. Indifference to other's acceptance or rejection is attained through patience and trust in Allah
4. Satisfaction with Allah is attained through contentment with what one has and submission to the will of Allah
5. Turning to Allah Most High is attained by gratitude to Him in happiness and taking refuge in Him in affliction

The foundations of the rules 

The foundation of all of this attainment consists of five things

1. High aspiration
2. Keeping Allah's reverence
3. Giving the best of service
4. Keeping one's spiritual resolves
5. Esteeming Allah's blessings

1. Whoever's aspiration is high, his rank rises
2. Whoever reveres Allah, Allah maintains his respect
3. Whoever's service is goodly is necessarily shown generosity
4. Whoever keeps his spiritual resolves continues to have guidance
5. Whoever esteems Allah's blessings will be grateful for them, and whoever is grateful for them will necessarily see them increased

The five signs of Sufism

A person will show five signs of Sufism

1. Seeking Sacred Knowledge in order to perform Allah's command
2. Keeping the company of sheikhs and fellow disciplines in order to see with insight
3. Forgoing both dispensation from religious obligations and figurative interpretations of scripture, for the sake of cautiousness
4. Organizing one's time with spiritual works to maintain presence of heart
5. Suspecting the self in all matters, in order to free oneself from caprice and be safe from destruction

1. Seeking sacred knowledge is vitiated by keeping the company of juveniles, whether in age, mentality, or religion, who do not refer from guidance to a firm principal or rule
2. Keeping the company of shaykhs and mureeds (disciples) is vitiated by self-deception and concern with the unimportant
3. Leaving dispensations and figurative interpretations is vitiated by leniency toward the self
4. Organizing one's time with spiritual works is vitiated by looking for more and more supererogatory worship
5. Suspecting the self is vitiated by satisfaction at its goodliness and uprightness

Curing ailments of the nafs (ego)

The principles for curing ailments of the nafs (ego) are five

1. Lightening the stomach by diminishing one's food and drink
2. Taking refuge in Allah Most High from the unforeseen when it befalls 
3. Shunning situations involving what one fears to fall victim to
4. Continually asking Allah's forgiveness (istighfar) and His blessings upon the Prophet (saw) night and day with full presence of mind
5. Keeping the company of him who guides one to Allah

Reaching Allah

One can reach Allah Most High by applying ten principles

1. Repenting from all things unlawful and offensive
2. Seeking sacred knowledge in the amount needed
3. Continually keeping ritual purity
4. Performing the prescribed prayers at the first of their times in a group prayer (and praying the confirmed sunnahs' associated with them)
5. Always performing 8 rakahs of the non-obligatory mid morning prayer (al-duha), the 6 rakahs between sunset and nightfall, the night vigil prayer after rising from the sleep (tahajjud) and the witr prayer as the last prayer before dawn
6. Fasting Monday's and Thursday's and the full moon days - 13th, 14th and 15th of the lunar month
7. Reciting the Quran with the presence and heart on the meanings
8. Asking much of Allah's forgiveness (istighfar)
9. Always invoking the blessings on the Prophet saw
10. Persevering in the sunnah dhikrs' of the morning and evening

Sources:
1. Al-Maqasid: Nawawi's Manual of Islam
2. Umdat as-Salik wa 'Uddat an-Nasik (Reliance of the Traveller)