If there is an ELD in the truck, what should a driver do to account for the miles driven under an emergency declaration?

There are three options

Use the “authorized personal use” (personal conveyance) function of the ELD to record all of the time providing direct assistance under the exemption.  Use of this function will result in the time being recorded as off duty and requires an annotation. 
Use the ELD in its normal mode and annotate the ELD record to indicate they were driving under the emergency relief exemption; or
Turn off the ELD, in which case the carrier would address the unassigned miles in accordance with the current regulation.