XML : Submitting website form data as XML via SOAP to .asmx

I have a website with a simple form collecting basic user data. I need to pass that form content to a 3rd party via SOAP method. They provided me with an XML template and a .asmx file URL to submit the data to.

In simple terms - what steps are necessary to submit this <form> data to the .asmx URL?

I am comfortable with PHP, Javascript and jQuery, but I have zero knowledge of SOAP - hence the newbie question. I will be very grateful to anyone for a step-by-step breakdown of the process or pointing me to the direction of a good tutorial on the topic.

Thanks so much!