If you get the ones with a soft rubbery insulator that slides off you can just solder them to the wire. Or just remove the insulator altogether, solder it on and then put some heat shrink tubing over the top.
As far as getting them to stick on properly in the first place a proper crimp tool is the best way rather than pliers. Pliers tend to crush the connector whereas a crimp tool will apply the correct pressure all the way around the connector.