Really you should go to the LBS, but if like everyone else you don't want to, use the time tested method, take off the old forks, put a lond screwdriver down the headtube and knock the cup out, going round and round to get it straightish. Then flip the bike over and do the same on the other cup. When the old ones are out, use a hide faced mallet to knock the new one in, making sure it goes in reasonably straight. When it's fitted, overtighten it and ride around a little, then slacken it off and check it after 50 miles or so.
