Most trailer nets have an elastic cord that you can easily tie around and under the trailer chassis, for example, by using the tie hooks.
Basically, it makes little difference exactly how you attach the net, as long as you make sure it is securely attached to the trailer, and more importantly, it will be around the load later.
Attaching the trailer net does take some time, which is why it is often tempting not to use it. Therefore, keep in mind that it is required by law to secure your load on the trailer and you should also think about the safety of other road users such as vehicles and cyclists!
Also read how to buy the best fitting trailer net for your trailer.