The Annual prune is best preformed in late February or early March in the UK. Just after the last frosts of the year and before the new growth of spring occurs.
Ideally you will need to reduce the mass by 25% to 30%, as shown in the picture. You can do this by hedge trimmers or manually with hedge trimmers. I would use the hedge trimmers to reduce and the shears to shape it into a tidy ball. Additionally you can see the main stems pertruding from the bush, these grow quicker and longer than the rest of the foliage. Once you have shaped it, reach down into the bush and trim down these main stems, probably 6 to 9 inches below the surface. This shall keep the shape for the rest of the season.
Now it is tidy, you can check for any dead stems and branches, usually they will die back to ground level, so trace the dead back and remove at ground level. If it just starting to die back, trace it down to the good wood, make a 45 degree cut just above the next live bud or lateral branch.
This annual prune will set the Abelia up for the season, it encourages new growth and in turn brings out the best foliage ressults.
Finally, its feeding time, Abelia's like a slow release fertilizer, 10-10-10 slow-release fertilizer preferably with an additional handful of Fish, Blood and Bone Fertiliser.
My appologies, I do not have any Abelia's in my garden, but I have just hard pruned a blackberry bush, it has a similar structure, I am not in anyway trying to compare, just to give you a visual aid for the finished product.