How do I claim my mother as a dependent and get the Fed taxes taken out of her SSDI payments refunded?

My mother has lived with us for over a calendar year. I plan on claiming her as a  dependent, but there is no way to claim her SSDI or the small pension she receives from my father who passed a few years ago. Do I file a separate tax form with her being claimed as a dependant? She has always had Fed Taxes taken from her SSDI payments and then claimed it for a yearly refund.