Look up cotter pin tool or cotter pin press or cotter pin and c clamp method.

or try this clever method suggested by a list member recently

tap tap tap tap...snug snug...TAP TAP...snug...TAP TAP...snug...WHACK...sn...

The snug part is never meant to draw the pin, only to take up the slack. The hammer part is never too hard, wild or out of control.

If this doesn't work your pins may be too distorted from being too loose too long. Find a shop that has a press. Better yet buy one here. http://bikesmithdesign.com/CotterPress/index.html