Adding Aria description to plugin on WordPress

I am a newbie to WordPress which I am using for a non-profit web site.
 
I've been scouring the Internet for several hours and have not been able to find step-by-step instructions (visual aids would be helpful) for how to add an Aria description to the "Seamless Payments" plugin for WordPress. Currently this plug-in has no form label and I do not know how to do this. 
 
Any pointers would be greatly appreciated so that I can make sure that our site and this form in particular is Accessible. 
 
Thank you for any pointers you can provide. 
 
Karen