Start by filing a Form I-130 visa petition for your unmarried adult son or daughter. However, the process could take a many years under the family-based 2B category (unmarried adult sons and daughters of LPRs) since the number of people who are allowed to immigrate under this category are extremely limited. And due to per-country limitations, if you were born in a country where many people are seeking to immigrate to the US, the wait could be off the chart.
If you later naturalize and become a US citizen, your unmarried adult sons and daughters will retain the priority date of the 2B visa petition and will automatically convert to the 1st preference category (unmarried adult sons and daughters of US citizens).
If the wait under the 1st preference category turns out to be longer, the Child Status Protection Act (CSPA) permits your son or daughter to “opt-out” of the conversion, and return to the 2B category despite the fact that you are now a US citizen.
We will be happy to clarify all intricacies of these cases. Please contact us for more information.