All I can say is give it more time, take it slower, back up a step, and keep trying. I don't know of a better way of training it, do you? Of course while the head loop is on you want to put in some effort to distract it from chewing it whether its with a treat, attention, or playing. However, if even a treat doesn't help, you definitely gotta back up to the material touch stage to show the bird more how it is harmless and earns it treats to be touching. It is also possible that the treats are ineffective. Either find more effective treats or make sure the bird is a bit more hungry prior to harness training such as removing all food from cage for up to 6 hours prior to training.
Also you said you read the blog post, be sure to check out the forum post as well as it is more detailed:
viewtopic.php?f=15&t=1329If you haven't already, watch the videos all the way through for more tips/tricks that I picked up for the process. Finally, don't worry if it's taking a while. I was able to do it in 3 days because I used an expedited training program but more importantly because my parrots were already trained many things before and know how to learn. If you have done less training before for other tricks, harness training will of course take longer. In the meantime, make a good carrier and start taking the parrot outside with you in the carrier so it can start getting used to being outside while you complete the harness training indoors. Good luck and let us know your progress.Don't worry and don't give up, you'll get it eventually.