how hard is it to install lowering springs

It's not hard at all. Just a bit time consuming and if you're not carefull, painful.
You can do it at home with some jackstands but if you have access to a lift the job is considerably easier. Basically, you'll need to lift the car and remove the wheels. Up front it will be easier if you remove the rotor and move the caliper out of the way. I recommend spring compressors so you don't have to worry about the spring accidentally releasing out of the car. The strut doesn't need to come all the way out, just disco'd at the top and let the arm drop as far as possible. If you have compressors, the spring can be lifted out easily. without the compressors, you might need a prybar get the spring out. The rear is easier. Just disco the shocks and quads and lower the axle slowly to let the springs extend fully. Then just take them out. Since the lowering springs are shorter, they just go right in and put everything back together in the reverse order. You'll need to get an alignment and install offset rack bushings or Steedas bumpsteer kit for lowered Stangs. It's not that bad, just don't rush through it.

Eric
 
I always use a shop for that one, there is considerable stored energy in the springs and it is a crappy job.

If you can swing it, pay the $150.00 most shops charge and get a burger while they do it.

As for an alignment, you are advised to drive it for several miles prior to allow for spring settle ( at least the Ford racing springs I bought did).
 
I've lowered 3 '03 Cobras. The first time was a MFer. The second and third were a PITA. I'd pay a shop to do it. Especially if you are only on leave for a short time.

Keith
 
I did springs on both my 86 Gt and my '01 Cobra. By far, the Cobra was more complicated due to the IRS. But even then, pretty easy. Read installs published in this and other forums - lots of good info which helps build confidence. Mark where the old spring pig tails sat in the cups before removing them so you know where the new ones go. You will probably have to use an air tool to loosen the top strut nuts - or drive it to a local repair place and give them a couple of bucks to knock them lose. The spring compressor is a very good idea (internal type) but I've done it without them - be VERY careful here either way, but especially if you don't use one. Good luck.
 
I haven't done springs on a Mustang yet but I've done them on more complicated suspensions and it's really not that hard time consuming yes hard no. Do it yourself if your aren’t intimated by it.