Documenting a decrease in income for IDR

I'm currently in SAVE forbearance but looking to apply for an IDR (I think I'm only eligible for old IBR). Last month, I left one job and started another in a different state, which meant that my spouse left their job as well. My new salary is higher than my old one. My spouse lost his main source of income but may do sporadic contract work. Since our household income will have gone down since our 2025 tax return I need to do alternative proof of income, but I don't know how to do that for my spouse's self-employment. Does anyone have any advice? Would invoices or receipts of payment suffice? For what time period would I need to provide proof? 30 days?