SFX can be divided up into 2 kinds of written form: an adjective/verb or an onomatopoeia. Adjectives and verbs are established words in whichever language you are using. Onomatopoeia can be made up words or clusters of letters that represent sound. Examples of adjectives could be: "clentch", "grit", "crackle". Examples of onomatopoeias could be: "fshwoo", "Butchwa", "grrraaa". I"m not sure if those exampes are very helpful but hopefully the concept assists you. Good luck!