The steps are as follows. Remove bolts from stem. Remove topcap from headset. Pull out fork. Remove brake attatchment. Install headset race on new fork. Cut fork to proper length. Install new starnut into fork. Slide fork into headtube after installing bearings from headset. Tighten topcap and stem bolts. Attatch brake.
These steps are not exactly in order. Installing the race on the fork requires a specialty tool but I've done it twice using various methods. Same with the star nut. You risk damaging the race and setting the star nut wrong with the shadetree method. If your not confident, take it to a shop. They can also recommend a suitable fork.