Firstly, there is the romanization which is used to write this language. This is the system I've used to transcribe it everywhere on this site, and it is in-universe one major way to write the language, mainly for European learners from the east. It is, however, secondary to the Antillean orthography.
The romanization has a relatively simple one-to-one correspondence of phonemes to symbols, and all letters are written exactly as their phonemes are in the International Phonetic Alphabet. An acute accent (í) is used to mark the stress when is isn't at the default position.
Next, and more importantly, is the Antillean script. It was originally developed to the north as an abjad for carving in stone, then adapted for use on clay tablets with a simple straight stick to make imprints. Here the script migrated south and the ocean-going people of the Oetasa islands adapted it into a syllabary for carving their own language in bark, which they had more access to than clay. They also used knives instead of simple sticks, as the medium was no longer soft and easy to imprint. Soon, ink and parchment spread from the Mediterranean west into Antillea, and the wood was replaced with pen and paper, leading to the modern script presented here.
There are thirteen grayed out spaces; these are syllables for which there is no character, and where they appear in words the method to write them is more complicated. The consonant-a symbol is taken (this is never missing), and the symbol for the correct vowel is placed above, like so:
mi
These thirteen characters come from the fact that when the writing system was originally developed it didn't have enough characters, so when it was adapted for use in Wetejo several had to be left out.
Syllabic-final nasals are very various. The na character is used when the nasal comes at the end of an unbound morpheme, like in ilón. The no character is used to represent a [ŋ] sound, or the /n/ phoneme when it's before /k/, inside of a morpheme. Similarly, ma is used to represent syllabic-final (but not morpheme-final) [m], which does not contrast with and is romanized the same way as /n/. ni is used for other syllabic-final nasals - that is, /n/ pronounced as [n] or [ɲ] at the end of a syllable but not the end of a morpheme.
[nn] and [mm] - that is, a cluster of a syllabic-final and a syllabic-initial nasal - are represented with a dot after the symbol for the syllabic-final sound. The next vowel is simply represented with its own symbol.