Iggeret HaRamban
אגרת הרמב"ן
Sefaria Community Translation
https://www.sefaria.org

Iggeret HaRamban

"Listen, my son, to the thought of your father, and do not forsake the teaching of your mother." (Proverbs 1:8)
Accustom yourself to always speak all of your words calmly, to every man and at every time. In doing so you will prevent your anger from flaring, which is a bad attribute in a man which may cause him to sin. And accordingly said our Rabbis, may their memories be a blessing: (Nedarim 22a) "Anyone who gets angry - all of Gehinnom holds sway over him, as it says: (Kohelet 11:10) 'And remove the anger from your heart, and take away the bad from your flesh', and 'bad' can only mean Gehinnom, as it says (Proverbs 16:4): 'And the sinner, he too, will have his day of bad'."
When you will have freed yourself from anger, the quality of humility will enter your heart which is the best of all good traits, as is written(Mishlei 22:4), "The return for humility is fear of G-d."
Through humility you will also come to fear God. It will cause you to always think about (Pirkei Avot 3:1) "where you came from and where you are going," and that while alive you are only like a maggot and a worm as after death, and before Whom you will eventually stand for judgment, the Glorious King, as it is written (I Kings 8:27) (Chronicles II 6:18) "Even the heaven and the heavens of heaven cannot contain You" -- "How much less the hearts of people!"(Mishlei 15:11),  It is also written (Jeremiah 23:24), "Do I not fill heaven and earth? says the Lord." 
When you think about all these things, you will come to fear God who created you, and you will protect yourself from sinning and thereby be happy with whatever happens to you. Also, when you act humbly and modestly before everyone, and fear God and (fear) sin, the radiance of His glory and the spirit of the Shechina (Divine Presence) will rest upon you, and you will live the life of the World to Come!
And now, my son, understand and observe that whoever feels that he is greater than others is rebelling against the Kingship of Heaven, because he is adorning himself with His garments, as it is written (Psalms 93:1), "The Lord reigns, He wears clothes of pride." 
What cause does one have for pride?  Perhaps his wealth? "The Lord impoverishes and enriches" (I Samuel 2:7). Perhaps his honor? It belongs to God, as it is written (I Chronicles 29:12), "Wealth and honor come from You." So how could one adorn himself with G-d's honor? And one who prides himself in his wisdom surely knows that God "takes away the speech of assured men and reasoning from the sages" (Job 12:20). Thus, all are equal before God, since with His anger He lowers the proud and when He wishes He raises the low. So humble yourself and G-d will raise you up!
Therefore, I will now explain to you how to always behave humbly. Speak gently at all times, with your head bowed, your eyes looking down to the ground and your heart focusing on God. Don't look at the face of the person to whom you are speaking. Consider everyone as greater than yourself. If he is wise or wealthy, you should give him respect. If he is poor and you are wealthier or wiser than he, consider yourself to be more guilty than he, and that he is more worthy than you, since when he sins it is inadvertent, while you act knowingly!
In all your actions, words and thoughts, always regard yourself as standing before God, with His Shechinah (Divine Presence) above you, for His glory fills the whole world. Speak with fear and awe, as a servant in the presence of his master. 
Act with restraint in front of everyone. When someone calls you, don't answer loudly, but calmly, as one who stands before his master.
Take heed to study Torah constantly, so you will be able to fulfill its commands. When you arise from your learning reflect carefully on what you have studied, to find a lesson in it that you can be put into practice. Examine your actions every morning and evening, and in this way every one of your days will be spent in returning (to God).
Remove all worldly concerns from your heart during prayer. Prepare your heart before God, purify your thoughts and think about the words before you utter them. 
Do this each and every day of your life, in all of your activities and you will not come to sin. This way all your words, deeds and thoughts will be proper, your prayers will be pure, clear, clean, appropriate and acceptable to God, as it is written (Psalms 10:17), "When their heart is directed to You, listen to them." 
Read this letter at least once a week and not less. Fulfill it, and in so doing, walk with it forever in the ways of the Lord, may He be blessed, so that you will succeed in all your ways. This is how you will succeed and merit the World to Come which is reserved for the righteous. Every day that you shall read this letter, heaven shall answer whatever arises in your heart to request, forever. Amen, Sela!