Whenever you are working wood with a chisel, it is essential that you keep the blade as sharp as possible. This will not only make your job easier, but will almost always result in a better finish to whatever you are paring. A blunt chisel blade will also be more dangerous to use than a sharp blade as it means you will have to force the blade and risk it slipping and injuring you (or at the very least damaging what you are working on).
To sharpen a chisel blade properly, you will need to buy a few things including a Oilstone, Honing Guide and light machine oil. Whilst these items are not expensive, and are useful to have in your toolbox, it may be cheaper to simply buy a new chisel if this is for a one-off job. Alternatively, some hardware stores will sharpen chisels for you for a small fee.
The first thing you need to do is prepare the Oilstone by pouring a little oil onto it and spreading it all over the surface. Let this soak in and repeat the process. Wipe off any excess oil and the stone is ready to be used. The Honing guide, which looks like a small vice with a roller on the bottom, then needs to be clamped onto the blade following the manufacturers instructions. The small flat face on the back (or top) of the blade should be flat against the Oilstone when the guide is rolled backwards and forwards.
Pour a little more oil on the oilstone and start to work the blade backwards and forwards in a slight circular motion. Keep the roller and blade tight against the oilstone with light pressure from your fingers. If your your Oilstone has a rough and a smooth side, and if your chisel is particularly blunt, start on the rough side and then switch to the smooth after a minute or two. Otherwise just use the smoother side. When you are happy with the edge, remove the honing guide and turn the chisel over. Holding the blade flat against the Oilstone, gently rub it back and forwards to remove the burr created by sharpening.