Crimping dupont pins is difficult for me, I'm old and not quite as steady as I was at age 25, and my eyes aren't quite as sharp as then, but, it can be done.

The large tabs at the end of the pin are for the wire's insulation, the smaller (usually square) tabs are for the conductor.
 

• Look at your crimper and decide which side has the curls, we'll call that the top jaw.  
• Holding the connector end of the pin insert the wire end into the crimper, with the open side next to the curls, and pull on the connector pin end until the insulation tabs catch against the ledge on the jaw.  
• Now, while holding the pin against the ledge, close the crimper (3 clicks on mine) until the pin is just held by the crimper.  
• Strip about 3mm of insulation from the end of the wire.
• Insert the stripped end of the wire into the wire end of the pin until the conductor is inside it's tabs, you should be able to feel when the insulation hits the conductor tabs.  
• Crimp.  
• Open the crimper, remove wire with pin crimped on the end.
• Lightly pull on the wire while holding the pin, the wire should not pull out!