Your festoon lights each come with their own power plug and each set can be connected together to create your required length.
First set of lights - remove the end cap on your first set and place to the side. You will find this at the very end of your festoon cable.
Second set. Remove the plug from your second set by unscrewing it from the festoon cable.
You will now have a set of pins and a set of grooves. Align these pins and grooves, push them together, and twist to secure them. Keep doing this with your sets until you have your desired length and ensure you have an end cap secured on the final set end.
Adding a dimmer or extension cable?
To add dimmer unit, follow the above steps. Instead of adding another set you will place the dimmer unit in between your plug cable and your festoon cable.
You will have the power plug cable > dimmer unit > festoon cable.
To add an extension cable, you will place this in between your plug cable and festoon cable.
You will have plug cable > extension cable > festoon cable.