Have you got the Ford (or bonky eBay equivalent) stereo removal keys? Should look like this:
Did the stereo your sister gave you come with the "cage" as well? Should look something like this:
Have you got the Ford-to-Sony wiring harness adaptor? (I had a Sony BT-3700U at one point, which I sold to The Arch Bishop when I got my Alpine) I can't remember if it needed an adaptor or not; I'm fairly sure it did. Halfords will have one if so, will be about £15. It'll look similar to this:
You should just be able to remove the existing stereo using the removal keys,
install the fascia adaptor (just clicks in place - well, the Ford one does anyway)
Plug in the wiring harness adaptor
Attach cables to the new stereo and put the rail adaptor on (your fascia adaptor should come with a clip that holds the head unit to a rail inside the dash), looks similar to this:
Then jam it all in the dash.