The fist step is to prepare the forms. The usual forms include I-485, I-130, G 325 A, I-864. Now, if an applicant wants to work in the US when the application is pending, he/she should file I-1765 form, which is free, and if she/he wants to travel, form I-131. If a joint sponsor is required, there will be two forms I-864: from the petitioner and the joint sponsor. Do not forget results of the medical exam which will be provided to you by the doctor in a sealed envelope. Make sure all the documents are signed and dated.
Prepare the Supporting Documents
The supporting documents required can be found in the instructions to the forms: they include evidence of the relationship; photos, birth and marriage certificates; copies of passports, visas, tax returns, etc. Follow the instructions to the form to determine what is required. Do not forget the fees! If you do not enclose the fees, the application will be rejected. Check the uscis website to make sure the fees are correct as they change.
Additional resources provided by the author
The best website to download the forms and instructions is www.uscis.gov.