They will twist right into the stock bulbs location. You'll have to crawl under the Mustang to get to them though, unless you converted to the GT style where they are in the upper grill.
Your owners manual will give this:
9145–GT or H11–Pony
(V6) or GT Cal Special
Googling H11 to LED gives this:
30 bucks on Amazon. Twist the old ones out, twist these in from underneath.