Don't: Bash the other parent.
You may have a long history of problems with the other parent that you want the evaluator to know about. You may have entirely valid concerns about the other parent's decisions or childcare practices. However, you need to be very careful about how you explain these concerns to the evaluator. This is not the time to go through a laundry list of complaints. One reason is that the evaluator is going to be looking at your ability to facilitate a relationship between the child(ren) and the other parent, and whether one parent is actively trying to alienate the child(ren) from the other. If you have nothing but bad things to say about the other parent, then it's going to seem like you are unwilling to facilitate such a relationship, or worse, that you are alienating the child(ren) from the other parent. Also, when you raise many complaints and some of them are minor, it makes your main concerns seem less important, like they are just another in long list of complaints, rather than a legitimate concern that deserves careful individual consideration.
You may also feel the need to get things off your chest. A custody and parenting time evaluator is not the person you should vent to. Remember, this person is going to be drawing very important, long-lasting conclusions about your parenting abilities and what custody and parenting time arrangement is in your child(ren)'s best interests. Also, a judge is going to listen to the evaluator's advice when ruling on custody and parenting time. If you need to get something off your chest, talk to a friend, family member, or a therapist instead.
Do: Be prepared to articulate your major concerns from the perspective of the child(ren)'s safety, stability, and long-term well-being. This means explaining why you believe something is either good or bad for your child and limiting your explanations to only those that are truly serious.
Do: Be prepared to talk about what you can do to facilitate a positive, meaningful, long-term relationship between the child(ren) and the other parent.
Do: Think about the future. You and your co-parent are going to be involved in your child(ren)'s life for the rest of your own lives. The present litigation is hopefully a temporary thing and you need to be able to see beyond it.
Do: Be prepared to tell the evaluator a few good things about the other parent.