Customs declaration

Can anyone direct me to the website where you can fill in your customs declaration online or print out a form? On Our flight with Air Canada they didn’t have any forms so the entire plane had to fill out the declaration after entering the customs hall. I have family coming and I’m hoping to make their arrival easier.
Thanks

This is the website with the form you're seeking. The form is called "Declaración de Aduanas para Pasajeros Procedentes del Extranjero".

https://www.anam.gob.mx/declaracion-de-mercancia/