add an apply gift balance button?

i would like to add a button on my checkout page for people to apply the amount of their gift balance to pay for their order total, i dont want this to be automatic just an option but i really dont know where to start, i guess the button should check their balance and insert into the box, they would still use the continue checkout button as normal, all i literally want the button to do is apply an amount that is available in that text box, nothing else.

could someone please help me with this?

thanks in advance :)

as far as i can see it uses this to display the gift voucher balance so i gues si use the same values and pass them to the text box am i on the right path?

$this->checkbox = $this->user_prompt . '<input type="text" size="6" onkeyup="submitFunction()" name="cot_gv" value="' . number_format($_SESSION['cot_gv'], 2) . '" onfocus="if (this.value == \'' . number_format($_SESSION['cot_gv'], 2) . '\') this.value = \'\';" />' . ($this->user_has_gv_account($_SESSION['customer_id']) > 0 ? '<br />' . MODULE_ORDER_TOTAL_GV_USER_BALANCE . $currencies->format($this->user_has_gv_account($_SESSION['customer_id'])) : '');

can anyone offer any assistance on this please?