Ideally use a headset press, which can be bought for under £ 40.00 really useful bit of kit to have.. or you could make a headset press tool yourself outta some threaded bar/nuts and flat thick spacers.. but what ever you do make sure it goes into the frame evenly and flat...
... but remember that it is easy to ruin the frame if you don't do it right the first time. the lbs won't charge much if you bring in the frame and the head set and usually the frame is worth much more than the labour.