You would install the software same as any other program.....
Basic workfow:
1) Chose which Free CAD Package you would like to use (if the one you want is not free you will have to purchase a license).
2) Download said software.
3) Install software.
4) ???
5) Get to work.
if you're looking for ways to use autocad, you will have to be a student to use autocad for free. you will have to register at the autodesk site, provide proof that you're a student. you can also download the trial version and use it for free.
just in case... read the forum rules here: http://www.tomshardware.com/forum/38307-63-read-first