The 12V solenoids come with long pre-tinned wires. These will be soldered to a cable with a 2.1mm barrel plug. This makes it easy to plug it into the DC jacks.
Using wire strippers, remove a bit of insulation for each wire. Tin the wires by adding a bit of solder. This helps prevent the strands of wire from fraying. Solder the wires from the solenoid to the cable with the 2.1mm barrel plug. The wire with the lined dashes is the VCC / voltage wire. Use pieces of heat shrink tubing to insulation the exposed connections.