So where do you start?
What we do as riders ultimately affects how our horse responds to a bitless bridle. You could put the bridle on the horse and turn it loose (without reins and under constant observation in a safe yard) and the horse would most likely go about its business with no problems.
In fact, that’s a good way to introduce the bridle and take note of how the horse feels about it before any pressure is applied. The cross-under style bridles will generally require the noseband to be fitted firm for safe riding as well as correct and efficient function of the design but for this exercise, keep it reasonably loose so the horse can eat, drink and be comfortable.
Be sure there’s nothing the horse can catch the bridle on if it rubs on a post.