Hi Peter The simple way I do it is to take the blade off and sharpen it on my bench grinder. Whatever grinder you use, be sure to keep the angle of the blade at about 60 degrees (there is a small angle jig you can get from most machine shop suppliers to quickly measure this). If the angle of the cut is too much (the cutting edge to thin), it will dull quickly and nick easily...unless it is high speed steel which you really don't want on a lawnmower blade.
To balance, I use an 8 penny smooth finish nail and balance the blade through the center hole, grinding down the side the is still to heavy. Not too scientific but seems to work ok.