deberán tener domicilio real inmediato anterior, con un (1) año de residencia, como mínimo, en la Provincia de Buenos Aires = must have actually been residing in the Province of Buenos Aires for a minimum of one year immediately prior (to appointment)

I add "to appointment" because this phrase works well in English like this, but stating something the residence is "prior to" is virtually mandatory. Or if elected, "prior to election", etc., but prior to something.