You'll need to document whatever needs to be forgiven. Documentation will vary based on the reason for inadmissibility. Did you lie in an interview? Did you lie about your marital status? Try to explain yourself in a sworn affidavit. Did you fail to disclose an expunged DUI? You'll need a criminal lawyer to try to unseal the record, get a certified court record and then seal it again. Did you leave the US because a loved one died? Gather the death certificate and again explain in a sworn affidavit why you left the United States.
US Citizen Spouse or Parent
In order to get a waiver, you need a US Citizen Spouse or Parent. Additionally, you'll need to prove that your spouse/parent will suffer EXTREME hardship if you are not granted residence. A Lawful Permanent Resident Spouse or Parent will also qualify. Hardship factors are described below. You'll need to collect a copy of your relative's US passport or naturalization certificate or alien registration card aka "green card".
US Citizen Child
A waiver cannot be granted solely on the basis of your relationship to your US citizen child. However, documenting that you have a US Citizen child or lawful permanent resident child will help you in the next section which is documenting extreme hardship to your spouse/parent. Understandably any hardship that will befall your child, will become an additional hardship to your spouse.
Put together a detailed household budget and collect receipts, invoices or bills for each line item. Collect pay stubs or W-2's to document income. Explain in detail what will happen financially if your household loses your financial contribution to the household. Identify in detail exactly what would happen. Would your spouse be able to make the house payment? Would your spouse be able to afford health insurance? Child care? What if you chose to leave the US with your spouse? What would the financial picture look like if you left to another country.
Health Related Hardship
Request complete medical, mental health and dental records for your spouse/parent. Highlight the diagnoses of any chronic conditions. Have your medical professional explain what would happen if your spouse/parent was unable to take the prescription medications that he/she is currently taking. Does your spouse/parent require future surgery or treatment? What are the consequences of not getting that surgery/treatment? Explain what would happen to your spouse/parent if they couldn't afford the medical insurance they now enjoy. Is there a history of mental health or mood disorders i.e. depression or anxiety? If so, get your mental health provider to provide a letter explaining how a separation could trigger a relapse.
Pillars of the Community with Deep Roots
Is your spouse/parent a pillar of the community with deep roots? Prove it. Collect birth certificates or identification documents for each and every family member living in the community. Has your spouse enjoyed stable employment for years? Document stable employment with W-2's and/or employment verification letters. How about community involvement in a church/temple? Get a letter from the pastor/minister/rabbi/imam documenting the length of time your spouse/parent has been involved in the faith community.
Do You Deserve to be Foregiven?
Explain why you should be forgiven. This section is more are than science and will depend on your reason for inadmissibility. If you have prior crimes that make you inadmissible, then document the fact that you have been rehabilitated. If you triggered the 3/10 year bar, give USCIS the reasons for your travel. Document that fact that you are otherwise a good person .e. law abiding, tax paying, community servant etc.
Get a Great Lawyer
An I601 Extreme Hardship Waiver of Inadmissibility is a very high bar to meet and requires that guidance that only an experienced and competent immigration lawyer can provide. The factors contributing to a successful waiver can only be found in the case law. You won't find it in the statutes or the regulations. The USCIS template denial says that any separation will cause hardship. An experienced lawyer can work with you to document that your separation would cause EXTREME HARDSHIP to your spouse/parent and maximize your chances for a successful outcome on a waiver application.