The best way to type Na'vi on your computer is to enable and use the “US (International)” keyboard layout, which is available on all major operating systems.
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Many people recommend using Alt codes: Alt+141 for ì and Alt+132 for ä (and more for Ä and Ì). This has several drawbacks.
Firstly, you'll have to memorize all of these random three-digit combinations.
Secondly and more importantly, it's slow and inconvenient. In Na'vi, ä and ì are letters just like any other, just like in English, q and x are letters just like any other. Now imagine that when typing q and x, you'd always have to use Alt codes. Super inconvenient! That's what it'd be like to use Alt codes for Na'vi.
Several people made custom keyboard layouts that replace keys for letters unused in Na'vi by ä and ì, ejectives, diphthongs, et cetera. If such a layout works for you, that's great; no need to change! But I feel that in general, the disadvantages outweigh the advantages. Mainly: you have to switch keyboard layouts every single time you want to switch between typing in English and Na'vi. With “US (International)” you can type English, Na'vi, and many other languages with the same layout without ever having to switch.