You can do it yourself, just make sure you're careful.
To take the old struts out of the car:
Remove the two nuts on the bottom (an impact wrench is your friend, and you'll need a mallet to pound the bolts out), and the three 15mm nuts on top. The strut assembly will come right out.
To take the springs out of the struts:
Rent a spring compressor from the local auto parts store. Use it to compress the springs. If you don't, the force of the spring can cause a lot of damage once you loosen the top nut.
Loosen the top nut in the center of the strut. IIRC, you'll need a torx bit to hold the strut shaft while turning the nut on top.
Remove the mounting plates. These tend to wear out over time, so replacing them might not be a bad idea.
Now's a good time to install new struts if your struts are really old or you're going for a severe drop. I'd also install a set of anti-pogo washers (they're pretty cheap, and help with ride quality with lowering springs)
Reinstall the new springs (you probably won't need the spring compressor with the lowering springs), then install everything else in reverse order.
To reinstall the struts:
The two bolts on the bottom of the strut have grooves to keep them from turning once they're installed. Once you can't push them in any farther, just thread a bolt on them and keep turning until they're fully inserted. Torque everything to spec, then recheck the torque after a couple of days of driving.
Lastly, get an alignment after a couple of months of driving. It'll take that long for the new springs to settle.