It is relatively simple
I almost got an interim EAD from Memphis. On the day I took the documents for my wife and I, they told me that my EAD had been approved by TSC and hence, they cannot grant me an interim EAD.
Submit a letter and a I 765 form, filled out. Also, attach a copy of the notice you received for EAD from TSC. As long as your ND is more than 3 months old they say they will give you an interim EAD.
Also, if you submit your form etc. this week (irrespective of which day of the week), you get your interim EAD card on Monday of the next week. I would say they are pretty customer friendly, based on my experiences there.
Good Luck
seebeekay