Adding a jquery text enlarger button to my site

Adding a jquery text enlarger button to my site

I'm using the Thesis Theme for wordpress (if that means anything to the potential answer to my questions) and I would like to insert a text enlarger/reducer button to my site to make it more accessible to my older clientelle.  I've found code out for this use, but I cannot figure out how to add the code in my custom_functions file and then place it within my html pages.
 
Can someone here please provide code for this and explan how to insert it where I need...??
 
Thank you very much for your time,
 
Charles A Smith