If you are from a Central American country, you must be a resident for three years, if you are from anywhere else, five years. If you marry a Salvadoran national, two years. While you are a resident, you must spend >50% of your year in-country (extenuating circumstances may apply).

Applying for residency can be complex, but essentially you fill out a form, provide several documents, and immigration issues you a one or two year visa depending on the program you apply for. More info here: https://escapetoelsalvador.org/residency-options/