Start small
Getting dressed for a walk can be easier when the harness or collar predicts good things and your dog has a choice to participate.
There is no need to rush. A predictable, manageable step is usually more useful than trying to solve the whole situation in one day.
Try this next
Show the item, reward a calm look, and build up from one brief touch to a short comfortable wear.
Notice what helps your dog stay comfortable and engaged, then repeat that version before making the situation harder.
Keep it kind and practical
Keep the next step easy enough that your dog can succeed. If you are worried about safety or feel stuck, pause and ask a qualified reward-based professional for help.