This essay won’t teach you to think in Ithkuil. I don’t even know whether it’s possible for a human brain or not. Still, in order to express ideas in Ithkuil, the first thing to do is to convert from thinking in English (or your native language) to Ithkuil. Ithkuil’s conceptual network is so unique and unrelated to conceptual networks of any natural languages, that expressing even simple words or phrases requires certain effort. Regardless whether you are translating from other language or expressing an original idea, you have to do the work of re-representing your (or the author’s) cognitive intent in Ithkuil.
Although it might sound intimidating, it brings certain benefits unlike a literal translation into Ithkuil. Most important, you have to think what you really want to say (from here on I’ll refer to the process of expressing your own thoughts in Ithkuil, so I presume you are the author of the message). Being honest with yourself provides a great psychological relief and unclouds your perception. Because of the idiomatic nature of our usual speech the meaning of the words we say is not the same as our cognitive intent. Some could view absence of predefined idioms in Ithkuil as a handicap, but I’d rather see it as freedom of choices that you have for expression. And that’s the next benefit. You don’t have to follow the narrow corridors of native languages’ semantic schemes when you use Ithkuil. For example, if you are speaking about food, you have a predefined verb choice in English. It looks simple at first, you eat solid foods, you drink liquids. Not without exceptions of course: you eat honey although it is liquid. Other languages impose different rules on you: Chinese people drink honey and brose, yet Chinese infants eat milk instead of drinking it. In Ithkuil the distinction between eating and drinking is not obligatory, you can always use the holistic word igral meaning ‘to eat and drink’, and if you go for complementary stems aigral ‘to eat’ and eigral ‘to drink’, the semantic distinction lies in the difference between the physical action of either chewing and swallowing or sucking in and swallowing, and not in idiomatic usage.
A model example of semantic analysis and transformation into Ithkuil is given in the narrative about the procedure of translating epigraph to “Anna Karenina” by John Quijada.
After the analysis you have to find the roots that you need for the translation. In fact, very few words of the original sentence usually require a root in Ithkuil. The core grammar (by which I mean all grammar except roots and derivation suffixes) can be used to express many subtleties of states and processes you want to speak about as well as put in your own attitude and personal feeling about what you want to say, and the better you know the grammar the more conciseness and semantic depth your phrases get. But don’t let the complexity of the grammar put you off. It turns out that for the sake of flexibility it is possible to express the same meaning with different grammar mechanisms. And even after you figured out which parts of your phrase can be handled by core grammar, don’t rush to the roots yet, first explore the vast variety of derivational suffixes that can handle a lot of meaning too. Most often a sentence will need only one or two roots.
Finally, you can go to the Lexicon to find them. It requires attention to get what you need, and if you fail to find the necessary root and stem at once, don’t forget that additional roots are published in the Lexicon Supplement (linked from the Lexicon page as a separate PDF file) and the newest sometimes can be found in Quijada’s comments on the Ithkuil subreddit even before they get into the supplement.
If there is no single root that can satisfy you, remember that the current version of Ithkuil also has a powerful root incorporation mechanism that can be used to account for a missing root even if it might get assigned at some point later.
And if after all this you are still sure that you need a separate root or an SSD derivation that has not been assigned yet, you can go and ask at the subreddit. Sometimes, Quijada gives out new roots for those in need, and sometimes he reveals roots he already assigned for use in his translations that has not been published yet.
After you’ve got all you need, you only need to assemble the words (and sentences) together and optionally write it with Ithkuil script.