1. Put the new wheel bearings in the freezer the night before you attempt this job - they will go really easily into the wheel when you do this.
2. Lay the wheel/tyres across two thick planks of wood which are thick enough so that the brake disks or sprockets don't touch the lowest surface. The wheel and tyre is entirely supported on the thick planks of wood.
3. Get a long piece of drift (a really strong tough piece of metal) and try and slide it through the centre of the wheel, past the top bearing and try and feel the inside lip of the bearing on the lowest surface. Hit the drift firmly and move around the bearing tapping it firmly as you go around. Stop and raise the wheel to see if the bearing is starting to slide out on the other side. Tapping it firmly or even more firmly but evenly so that the bearing exits the wheel all the way round.
4. Save the removed bearing and spacer put them to one side.
5. Turn the wheel over and wrap some tape around the drift where it touches the inner of wheel where you have just removed the bearing to protect the inner edge of the wheel where the new bearing will finish, repeat tapping firmly on the opposite bearing.
6. Clean up the outer edges of the wheel where the bearings will sit with a toothbrush or some very fine wet and dry paper if necessary, but don't remove anything more than any light corrosion - be very gentle here on this surface.
7. Go and get the new bearings from the freezer, wrap one in a rag to keep it cold.
8. Insert spacer
9. Press the new wheel bearing up against the wheel and use the old bearing to sit on top of it and hit it gently all the way round and start driving the bearing into the wheel evenly all the way around. Only hit the old bearing on top of the new bearing
10. Use a piece of flat wood slightly bigger than the diameter of the bearing and start to hammer home the bearing evenly so that it goes into the wheel straight - DO NOT HOLD THE WOOD - you'll see why. Stop when the bearing is flush all the way around the wheel
11. Turn the wheel over and check that the spacer is inserted inside the wheel.
12. Repeat as above driving the second bearing into the wheel on the other side using the old bearing and the piece of wood.