In order to write a soliloquy, one must first understand what a soliloquy is. A soliloquy is, according to eNotes,
a speech delivered by a character in a play or other literature while alone, or an utterance by a person who is talking to him/herself, disregardful of or oblivious to any hearers present.
Therefore, one must take into consideration things which the character they are creating the soliloquy for may be concerned with.
As for writing a soliloquy for a character in Macbeth, one would need to consider what is being left out of the play, what needs more depth, and/or how may the writer defined the actions of a character through a soliloquy creation.
Therefore, to begin, one must figure out what exactly they understand about the play and where they are lacking in understanding. A soliloquy could help the writer to define their own interpretation of the action or mind of a character so as to develop a better understanding.
Examples of soliloquy starters are:
1. Lady Macbeth explaining her madness and where it came from.
2. Banquo's true feelings about the prophecies and Macbeth's assumed role ion the murder of Duncan.
3. Malcolm's fears about being next in line to be murdered.
4. Duncan speaking about his inability to truly judge those around him well (given both the Thane of Cawdor and Macbeth were not truly good people).
If you choose to use any of these suggestions be sure to provide needed knowledge for the "reader." Look inside who you think the character honestly is and describe their feelings according to your interpretation.