Use a crimping tool. It compresses the contact barrel tightly around the cable wires so that all of them are pressed tightly against each other and the inside wall of the contact barrel. This requires that the stripped cable be inserted all the way into the barrel of the contact, and that the contact point is centered in the crimping tool.
If you don't have a crimping tool, the alternative is to solder all cable strands within the contact barrel. The right proportion of solder is essential if this procedure is used. Use a quality solder in wire form with a rosin flux core. The cable strands should be separately fluxed with rosin paste, and the contact should be held in a vise with the barrel end facing up. Apply heat to the outside of the barrel while the solder flows in beside the wire strands.