First of all, use the search function at the top of the page as this type of question has been asked a thousand times.
Secondly, to answer your question:
What kind of springs did you get for $99? If they're cheap pieces of crap, you will regret it.
It may or may not take you the better part of the day in order to install new springs depending on how many people you have helping, the type of tools you have, and many jacks you are using, and if you bought shocks/struts and C/C plates.
Really, all there is to it, is jacking the car up, supporting each of the
control arms as you go, and...
Lower the
LCA's down pretty far, unbolt the shocks struts.
Compress the spring, use the spring compressor.
Lower
LCA till spring falls out.
Unbolt the other end of the shock/strut.
Replace shock/strut with new ones.
Put new spring in, raise
LCA into place, and bolt the shock/strut back in place.
Depending on how low you go, if over 1.5"-2", you need to put on caster
camber plates. Read directions that come with them for the how-to.
Then go get an alignment.
People will tell you to drive around on the springs for a week after an initial alignment, and then go get another alignment, which just ins't true. This, being that if your springs didn't settle all the way within the fist 20 minutes of them being installed, then their tensile strength is crap, and you will forever be getting alignments until the car finally bottoms out.
A how-to...
http://mustangworld.com/ourpics/fcar/steedaspring1.htm