Explain What You Want
Begin your affidavit by copying the caption from your divorce petition. It should include your name, your spouse's name, the name of the court that has jurisdiction over your matter, and your docket or case number. In your first paragraph, explain to the court that you're filing the affidavit in support of your motion. You can then explain what you're asking the judge to order.
Explain Your Reasons
You must also explain to the court why you're making your request. For example, if you're asking for an order giving you temporary custody and establishing a visitation schedule for your children with your spouse, you might want to tell the court why you think you should be the custodial parent pending your divorce. Give examples to back up your reasoning, but try to keep your statements fact-based rather than use conjecture. For example, you can say, "My spouse does not use a car seat when driving with our baby," rather than, "I don't think my spouse always uses a car seat." However, you must be sure that what you're saying is true, and you can attach exhibits to your affidavit to prove your statements. For example, after you explain that your spouse doesn't use a car seat, you can say, "See Exhibit 1," then attach a photo of your spouse driving your baby without one or a notarized statement from someone attesting to this.
Affidavits are made under penalty of perjury, so when you finish explaining everything to the court, you'll need a final paragraph stating that you're aware of this. You can say that everything you've said is accurate and true, and that you're signing the affidavit with the knowledge that if you've lied, you're subject to punishment. Your spouse or his attorney then have the right to produce your affidavit as evidence at trial when you're litigating a permanent custody arrangement at the end of your divorce proceedings.
You may have to draft more than one affidavit as part of your motion to the court. Your first one supports your notice of motion, and after you file it with the court and serve a copy on your spouse, he or his attorney have a legal right to respond. Your spouse can file his own affidavit denying or rebutting what you've said in yours, or he can even file a motion of his own, called a cross motion, asking for his own relief. For example, he might respond not just by saying you should not have custody, but by asking the court to grant him temporary custody instead. You have the final word, however. You can file another affidavit rebutting what he said or explaining why the court should not grant his cross motion. Court rules can vary somewhat by state, but typically, your second affidavit is limited to addressing what your spouse said in his – you can't present new arguments.