Quick answers to your questions:
1. You'll need to attach a copy of the Final Judgment expressing the support obligation you are attempting to modify.
2. If you don't know his income, propound a Demand for Mandatory Disclosure from him - he'll be required to provide you his financial information and a new Financial Affidavit.
3. You do not need to file the Child Support Guideline Worksheet until after you have his information so you can run guidelines.
4. If he does not provide the information you can compel him to provide it by motion and seek your costs/fees for same.
5. You can also propound Requests for Admissions seeking to have him admit how much he makes, or seek third party discovery from his employer, etc. There are many ways to find out his income.
6. The modification should be considered retroactive to your date of filing - if you plead for that in your petition and the facts supporting your petition existed at the date of filing.
The answer provided is a general answer to a general question - and should NOT be construed as my giving specific legal advice, nor does it create any attorney/client relationship or expectation of privilege - and should you have specific questions you seek to address, then I encourage you to seek a consultation with an experienced Family Law Attorney in your area who may discuss your case with you and give you specific advice to assist you.
Visit the link below for some forms and assistance with what you'll need to file along with the Supplemental Petition. Include everything that it asks for. You don't need to know your husband's income when you file - he should be providing that to you before the final hearing, through the discovery process. Once you have it, you can complete the child support worksheet. The modification, if granted is typically retroactive to the date of filing the petition so it's in your best interest to get it filed as soon as possible.
This response is for general purposes only and does not establish an attorney-client relationship. You should contact an attorney to fully discuss your issues.