First, measure the length of the steerer tube and cut accordingly to the length you need.
then,
1) Remove brake from old forks
2)Remove front wheel
3) Unscrew your top cap bolt and remove bolt/top cap
4)Slide off stem/bar
5)Slide out forks and remove the crown race.
6)Fit the crown race to the steerer tube on your new forks
7)Slot steerer tube through headset and attach stem and necessary spacers
8)Attach brake and wheel
9)Fit top cap and hey presto, go ride!
Hope that helps