I use a bit of soft wood(2x4) and a hammer-just like tellyho says.One thing that makes it easier is to take a file,and then some fine sandpaper to the headset.File,sand the parts that are being hammered into the frame.You want to take off a tiny bit of metal,so the fit isn't so darned tight.Check the fit first-try pushing the cups in by hand,and see how deeply they go in.If they barely go in,attack with the file.Then use sandpaper to smooth out any file marks.Oil or grease the parts to make then easier to hammer in.
Get someone to hold,and back,the frame.If possible brace the opposite side of the head tube on soft wood also.
Obviously,this isn't the sort of thing I would do on a zillion dollar bike, but it is fine for most bikes.
Luck,
Charlie
PS Be very careful with the hammer-no big swings-12" arc is plenty.You don't want it to slip and hit the frame.You can also use a 2 foot length of 2x4 for a hammer.Use a helper to hold/steady the frame.