sitelen sin is an alphabet (technically an abugida/alphasyllabary) for toki pona, based on sitelen pona - type most words as you would write them in Toki Pona, with a few exceptions.
If a word starts with a vowel, add "x" before the vowel. This acts as a spacer so the vowel gets rendered correctly. If a word has a syllable that ends with -n, instead of typing "n", type "`" this adds a diacritic to mark the final "n" properly.
Separate words with dashes, not spaces.
The numeral system is as follows: 1,2,5 have their expected meanings. 6-9 are "sijelo" (20) "kulupu" (100) "mute" (500) and "ali" (2000) in a Roman numeral-type system (placing a number word before a higher one subtracts it)