the easiest way to fit the bearings is a small block of wood and a heavy hammer. plenty of small taps on the wood on opposite sides. you must be reasonably accurate with the hammer, if the bearing 'turns' you may stretch the metal of the head tube and end up with an oval hole.
if you prefer a more 'delicate' approach get the fattest threaded rod you can and pass it down the centre of the head. then put a plate at each end and solwly tighten, fitting one bearing at a time.
provided your not a compete animal the hammer and block approach is simple and quick.
when removing the bearings, try to get them to come out parallel with the head tube, or again you may bugger the part of the frame where the bearing sits. lots of small taps on opposite sides.
bullet350