Replacing them are quite straight forward only that you might have to fight with them to put them on. Doesn't matter where to put on first - the outcome is about the same. Some lubrication might help too for example pure dishwasher fluid (don't mix with water). Using grease or oil might deteriorate the rubbers later. You need a long screwdriver (about 1 foot) to assist you. Do this on cold exhaust. Alternatively a muffler shop might easily install these. They have assortment for most cars.

While you're at it replace all rubbers even those old intact ones. With new rubbers on you might be surprised how quiet the car could be (less low freq vibrations).

Regards,
Amarin.