Where should I begin the following assignment?Macbeth has hidden himself in a castle chamber to write a letter to the local Scottish advice...

There are two things that are needed to begin this assignment. 

First, you need to collect examples and evidence from Macbeth that detail Lady Macbeth's changing behavior. Look for times when she is acting especially dark (like in her soliloquy in I.5), especially controlling (like in her reaction to Macbeth's ghost-sighting in III.iv), or especially manipulative (like in her scene with Macbeth in I.iii). 

Watch for how Lady Macbeth uses various tactics (like anger, seduction,...

There are two things that are needed to begin this assignment. 

First, you need to collect examples and evidence from Macbeth that detail Lady Macbeth's changing behavior. Look for times when she is acting especially dark (like in her soliloquy in I.5), especially controlling (like in her reaction to Macbeth's ghost-sighting in III.iv), or especially manipulative (like in her scene with Macbeth in I.iii). 

Watch for how Lady Macbeth uses various tactics (like anger, seduction, guilt, and pride) to control what Macbeth thinks and does. 

Collect these examples and keep them in a place where you can easily reference them while you write. 

Second, you need to understand what a soliloquy actually is. Luckily, Macbeth is full of them! A soliloquy is when a character speaks his or her thoughts out loud, especially when that character is alone onstage. It's essentially a monologue spoken to oneself or to God. Check out Macbeth's soliloquy in III.i, after he sends away all of the other characters onstage. It begins, "to be thus is nothing."  

Once you're set with both of those things, the only remaining task will be to turn your examples into a soliloquy! Pretend you're Macbeth ranting to yourself about your domineering wife.