Until thou hast accomplished these three, thou shalt not write any Immortal character, Power or Undying, in Arda or in Eä:
 
 

I. Let thou have spent the entire night to sunrise talking thyself hoarse (most preferably without the aid of ale) of poetry, history, philosophy, and song.

II. Let thou have been forced to choose between on the one hand, food, and on the other, book or music, -- and have chosen rightly.

III. Let thou have at risk of life, limb, or even mere considerable personal discomfort (such as sub-zero weather at the depths of a midwinter night) spent length of time outdoors to view the stars, the mountains, the forest, or the sea.

Alike must thou this injunction take well to heart:
 
 

IV. Thou shalt not write Arda Immortals until thou canst in all seriousness say -- That was but a hundred years ago -- and mean it, nor find anything at all strange about the saying thereof.

V. Thou shalt have the Sea-Hunger in thy heart, and the love of the Lonely Places.

Furthermore:
 
 

VI. Thou shalt not write warrior characters until thou hast at bare minimum swordfought thy siblings with sticks and borne the bruises to prove it. (This goeth thrice if thou wouldst write a Mary-Sue.)

And -- hast enjoyed it.

Moreover, let thou be at ease bearing arms, so that thou thinkst naught upon a blade at hip or in hand or boot, neither thine own nor others.

VII. If thou wouldst Wood-Elven folk describe -- then first shalt thou be at ease in trees. By preference to other worked wood, and those implements of torturous artificiality, "park benches", and without regard to the gaze of the curious and strange. If thou canst not even imagine crossing water without care of thy shoon, then most definitely canst not write a Wood-Elven Prince (or Ranger).

VIII. If thou wouldst Noldor or Dwarven characters write, then first let thou have worked on the makings of thine own hands to exclusion of all else, not food nor sleep nor entertainment to distract thee. If thou hast not spent time upon times in contemplation of a single stone or a working of metal, and but begun to sense the wonder of it,  best thou not dare to ape such folk in thy writings.

IX. If thou hast not spent so much time in hasty travel, unsupported, without assistance, bearing all one's own gear and goods with one's self upon one's own resources that a full change of light and dark has passed, without proper shelter or certainty of accomplishing such, nor free access to water and food, then thou hadst best think well and hard ere daring to write an Elven-force flying cross country -- or a Ranger.

X. Afflict not thine own neuroses nor preconceptions nor prejudices nor likewise the ailments of thine own Age upon the Ancient of Arda -- they have their own problems enow. Neither shalt thou impose thy Race's own biological imperatives on them -- of necessity theirs are needs directed else. Exercise thy imagination...

"By age of sixteen" -- I had done all but the ninth of these things, and near to the ninth (which subsequently have accomplished) so I enjoin not impossibility. Yet if it be so that circumstances conspire gainst thee, that for reasons of wealth or kin forbidding or other absence of resource, then let thy mind and fancy and seeking thought delve out of the shadows of book and dreaming such images that all these things, though untouched, be nevertheless full real to thy soul.

I confess that I am somewhat of a loss as to what to recommend, if one has not the Sea-Hunger and the love of the Lonely Places, as I was born with both and with the heritage of both, and have literally never known a time when I have not dreamed of tall ships and submerged cities and mountains above and below the sea.

But I think that the same sustenance that fed those when I was long away from both sea and hills -- photographs of great clarity and National Geographic articles and films of the high seas and sea songs and old tales and suchlike -- would perhaps serve as tinder at least in which some spark might take hold, if not as coals to ignite the longing.

And I truly believe that these things are more important than any amount of technical research, scholarly delving, or smoothness of style -- that without even the shadow of these things in the soul, no author can write well Immortals of any subcreated world, our own or Arda: what will result are mundane though super-powerful soap opera characters -- not the Powers, not those so rightly called 'fey', not any being ancient and wise and strange beyond our ken, yet still near enough for words.

As for the tenth, that is, in its own right, the most powerful and unbreakable mandate -- unless you're trying to write "Friends" of Middle-earth ...