Listen to the traditional mariachi music of Mexico. You'll be able to find plenty of recordings online, at various music download websites and video sites such as Youtube, or at a record store. Absorbing a variety of mariachi music will allow you to understand the characteristics of the music, including pacing, rhythm, melody and mood.
Understand the instruments involved in a traditional mariachi ensemble. Many people confuse other Mexican musical styles with mariachi music. Mariachi ensembles usually include violin, guitar, trumpet, guitarrón, vihuela and sometimes harp. The guitarrón is large, six-string acoustic bass. The vihuela is five-string guitar-like instrument similar to the guitarrón with ukulele tuning.
Consider which instruments are included in the specific mariachi group you choose to write for. If you aren't writing songs with a particular ensemble in mind, try to include parts for all instruments normally included in an ensemble. A smaller mariachi group will be able to adapt your piece according to its needs.
Decide whether you'd like your mariachi piece to be instrumental or sung. Most mariachi songs include a vocalist, often with multiple instrumentalists doubling as vocalists and singing in harmony. The lyrics of mariachi music typically deal with love and affection, though topics such as sadness and longing are sometimes covered. The lyrics you write could be in English, though the majority of mariachi music is written in Spanish.
Choose a meter for your song. The many variations of mariachi music are grouped into different rhythmic meters. The jarabe and huapango are typically in 6/8 time, while the cumbia and bolero ranchero are usually in 4/4. Perhaps most popular are the mariachi styles in 3/4, such as the valses Mexicanos and corrido. You can review mariachi sheet music to get a sense of your chosen meter.