show pictures for me to give a step by step instruction sugestion.
It depends on what you have and what you need it to do:In general a hole in the gear is put over the shaft - a set screw or woodruff key is inserted to stop the gear turning on the shaft.IF the gear and shaft are different diameters then you may need to make buy a bush to go between both.
Is the motor petrol or electric? How big is the motor? Does the shaft have a keyway?More info please.
Uh... I dunno. Does it just so happen that there is a circular hole in the center of the gear with a diameter that's almost the same as the diameter of the shaft on the motor? Because if those conditions were true, then you could just push the gear onto the shaft. It would also be neat if this gear did not need to transmit more torque than that torque which will cause the gear to slip, and turn on the shaft, instead of the gear and shaft turning together. Also supposing there is this problem with the gear slipping on the shaft, then it would be neat if there were some mechanism for to tighten the gear's grip on the shaft, like a set screw, or something like that.http://en.wikipedia.org/wiki/Set_screw That's assuming that static friction alone is not enough to keep the shaft and the gear stuck together.