When you have negative information to present, do so in an objective manner.
No doubt you'll have some information, whether verbal or written, that bears negatively on your spouse. (At least, you had better hope you do.) Presenting it without "slamming" your spouse is important- if the evaluator gets the impression that you're on a crusade to smear your spouse, it'll reflect negatively on you as well.
When you have negative information to present, do so in an objective manner. Most documentation speaks for itself, such as copies of bad checks she's passed, letters she's written that may slander you or unpaid bills. Even if the significance is obvious to you, it may not be to the evaluator. Always include some kind of cover letter briefly explaining what each document is and it's relevance to the evaluation. Examples might be:
The enclosed W-2 forms are included to show that my ex-wife, Janet, has had numerous jobs in the time that I've known her and indicate some instability in her employment.
The letter shown here was written by my ex-wife, Janet, and contains statements that are misleading or are untrue. I think this shows that she is willing to lie about certain events to gain sympathy or to get something she wants by distorting the truth.
In the second example, provide a line-by-line description of whatever statements that were untrue, and if at all possible back up the description with proof, whether by reference to other documentation or a collateral contact.
Verbal information is often more difficult to present without sounding like you are placing blame. Make every effort to be objective when communicating verbally with the evaluator, if necessary, practice what you want to say until you can get the information across in a neutral manner.
Instead of: "She was screwing around with some guy she met at that cowboy bar"
say: "It's my understanding that my wife was having an affair with someone she met at Jerry's Tavern"
Instead of: "She could never pay a bill on time to save her life, she was always late, no matter what"
say: "She has a lot of difficulty managing financial obligations like utility bills, the bills would often come back as 'late' or unpaid."
Instead of: "She's pissed off everyone she's ever known and can't keep a friend."
say: "She has a lot of difficulty maintaining good interpersonal relationships."
Instead of: "I am upset" say "I am concerned".
Instead of: "She is unfit", say "I think she may have a tendency to become overwhelmed at times."
Instead of: "I'm frustrated with the "system" state that "It is so challenging to balance the scales when coming to an agreement".
Hopefully, you see the difference. The second set of statements convey the same information as the first, but do it in a more clinical and objective way. By using the second set of statements you remove yourself from the "blaming" position to that of a "reporting" position. This may seem like a small thing, but it isn't.
Evaluators look to see how much animosity or hatred you harbor towards your spouse, and any overt amount will be "scored" against you overall. One of the things evaluators must judge for the childrens sake is "who can support the least conflicted relationship with the other spouse". In plain English, "which spouse is least likely to talk negatively to the children about the other spouse". This plays a big part in who is more likely to be considered by the evaluators as the best candidate for Primary Parent (custody).