If you get these elements of composing right you will be on your way to writing convincing and musical melodies in the style of Haydn and other WCT composers. Here is a Style composition basic crib you can print off to remind you.
WCT Composition PRESENTATION GUIDE (WAGOLL)

It is important to listen to music in this style in order to copy it successfully. Here is a Spotify playlist of WCT Quartets to get you started: WCT String Quartet Playlist