The following is an explanation of the typical way to write on an o-bon lantern but there is no definite way.
Typically, two different phrases are written: one on the front and back sides and one on the left and right sides. Or, you can also write one phrase on the front side, a different phrase on the back side, and leave the right and left sides blank. It`s up to you.
Traditionally, the lanterns have been used to mourn for lost loved ones or one`s ancestors so people often write the Buddhist memorial name (a name given to a person after their death) of their lost loved one or other related words or phrases. However, recently, many people have begun to write wishes for “business success” or “family safety” on the lanterns so they can be used for other purposes besides mourning the dead.