Creating a clickable button is a very simple bit of HTML coding:
<a href="[affiliate link URL]"><img src="[button image URL]"></a>
The information you need is her affiliate link, which is available from her account. It isn't something you can look up yourself but if she's provided it to you then you're good to go.
We provide that link to the affiliate in a set of HTML code that is ready to go to make it into a clickable text link, all you need out of that code is the address in quotation marks, which would go into the same place in the code above.
She might be able to just get that URL herself and give it to you, or she might give you the entire block of HTML code that contains it. This help page explains exactly what to look for in the code:
As for the button image URL, you will need to provide that yourself by uploading an image and providing its address where indicated. Be sure that the address starts off with http:// or else it will not work as expected if the code is used anywhere else but the same site where the image was uploaded.