Early Spring is the best time of year to dig your hostas up and divide them. Use a garden fork and loosen the soil gently right around the hosta clump then gently place your fork in below the clump and lift it up give it a shake with the fork to loosen the soil.
I prefer to wash the clump this serves to make it easier to see the main eyes (growing points)
of the hosta plus it also cleans the roots from weed seed and any slug eggs that might be tucked in around the eyes.
If your clump is large say 60cm in width the best method to split the clump is two forks placed back to back in the centre of the clump and pull apart. If the clump is smaller I use an old putty knife it is ideal for placing in between the eyes and cutting down through the side of the crown then pull the division apart. Make sure to clean and sterilise your tools before each division to stop the spread of any disease. Once you have divided the clump up I would soak the divisions in water with a weak solution of a garden fungicide for a few seconds this helps stop root rot and any cut end of the crown dap Flower of Sulphur on it.
Plant your division right away do not let the roots dry out. You can also pot the extra divisions up until you have time and room to replant in the garden or grow on in containers. Do not feed at this stage with a high nitrogen fertiliser you want to encourage root growth first so a good organic slow release fertiliser is ideal at a low rate. Prepare your ground well by double digging and allow at least twice the size of your clump you are going to plant as the size of hole to make to plant the division in.
See question on How to Plant Hostas.