Here are some general guidelines for responding to a grievance against you. Obviously, your specific case may require a different approach. The Office of Disciplinary Counsel also has a webpage about responding to a grievance you may wish to review. If you do so, I recommend also reading the applicable rules. The following is not intended to be legal advice nor should it be considered to be a substitute for personal legal advice, which can only be provided after consideration of the specific facts at issue.
Check your malpractice policy
Your malpractice policy may cover defense of grievances, often under "supplemental coverage." Under some policies, there is no deductible for this coverage (although the amount of legal fees covered is limited). You may also want to check whether you need to report the grievance to your carrier and if so, the deadline to do so.
Note deadlines and comply with them
The cover letter from the bar will give you a deadline for responding to the grievance. Under ELC 5.3(h)(1), the bar cannot take any action against you for not responding until 33 days after it has mailed you the request for response. If you don't respond within the 33 days, after sending you a warning letter giving you ten days to respond, the bar can subpoena you for a deposition at your expense and charge you $500 for their time.ELC 5.3(h)(2). The bar can also seek to discipline you for failing to cooperate if you don't respond as required under the rules, so it's very important to take the deadlines seriously.
Remember your audience
To you, the most important facts may be about how successful you were in representing your ungrateful client and how difficult the client was, but if the client is complaining about not getting an adequate settlement statement, the bar isn't going to care very much. Your response should provide sufficient information about the case so disciplinary counsel understands the context, and then address each alleged ethical violation and provide a response. Tempting though it will be to describe in detail your client's many wrongdoings, including too much of this information makes it look like you may have something to hide.
Review your file and other documents
All too often, lawyers write responses to grievances based on memory alone, failing to realize that their memories may not be accurate. Before you send a response to the bar, take the time to review your file, relevant court documents, etc. to make sure that you are remembering the facts correctly. You don't want to lose credibility with disciplinary counsel because your response is inconsistent with the relevant documents, or worse, have disciplinary counsel conclude that you were misleading in your response.
Have a friend help you
If you are representing yourself in the grievance process, you may find it helpful to have another lawyer review the grievance and draft response to make sure the tone is appropriate, all points are adequately addressed, and you aren't unwittingly admitting to a rule violation you had not even considered. If you can't bring yourself to do this, you may want to write the response and put it aside for a few days and then reread it to see if it needs further editing.
Consider hiring a lawyer
If you think you may have violated an ethical rule, are uncomfortable representing yourself, or are concerned that you may be unaware of rules implicated by the grievance and/or response, you may want to hire a lawyer. This can help you assess whether you have done anything wrong, and if so, how serious it is and what the likely outcome will be. Some lawyers chose to hire counsel as soon as they receive a grievance and others wait until further in the process. If you receive a letter recommending a hearing, it is advisable to consult with counsel as soon as possible.
You should also be aware that the bar has a diversion program for "less serious misconduct." This provides an opportunity for some lawyers to resolve grievances without public discipline, but is only available up to 60 days after service of the formal complaint. ELC 6.1.