Need help installing new prop!!!

I am trying to get my new prop on my boat but cannot get the thing to go on all the way. It goes about 3/4 the way on the shaft and stops. I have tried all I can think of to get it all the way on past the threads. Does the prop key go in the prop before putting on or on the shaft then slid the prop on? Any help you all can give me would be great so I can go boarding tom. Thanks