Writing user scripts requires knowledge of JavaScript. The tutorials on the GreaseSpot Wiki should give you a running start on JavaScript both in general and as it relates to user scripts. Inspecting others' scripts can be helpful. You may be interested in Greasy Forks regler för publicerade skript and Skriptets metanycklar, lästa av Greasy Fork. If you want help developing your script, feel free to fråga i forumet.
To post on Greasy Fork, you will need to create an account. From your användarsida, follow the link to publicera ett nytt skript.