Harness training is very similar to slip-lead training.
Do not get frustrated, remember that your dog has never experienced a harness before and rushing it will only send them backwards.
You are open to the types of harness you use, however, ensure it is well fitting, is not sliding around on the dog as dogs can escape harnesses in seconds.
Practice desensitising your dog to the sounds of the velcro and clips before you start luring them to place their head into it.
Use the guide above to get the dog used to putting their head in and resting the harness on their back.
Over time clip the harness and take it off, repeat.
Once the dog has the harness on you can leave them to potter about the house and garden in it so they get used to the feel. You can also attach a tralining line to the harness too and allow them to get used to the feel of it.
Harness training can take a good amount of time. Again, it is not a race. Do not set yourself time guidelines, just work at the dog's pace.