Strictly speaking, it is a private devotion, and you can wear it in any way you see fit, as long as it conforms to standards of piety and modesty. However, if you wish to participate in the prayers, blessings, and spiritual benefits attached to it, you would have to follow whatever “rules” are prescribed for it. (Keep in mind that you do not sin in the least if you fail to “follow the rules”.)
The only thing I would say is that it is better actually to wear it, either 24/7 or at least during waking hours. I only say this because I have heard of people carrying it with them, or other such things. Again, it is a private devotion, but I fail to see how carrying it in one’s pocket, hanging it from a rear view mirror, and so on, has any spiritual benefit.
I wear the Brown Scapular 24/7 and only remove it for hygienic or safety reasons (such as if I am working on a motor or piece of machinery and it could fall out of my shirt and get tangled). Sleeping in it takes some getting used to, but after 21 years, I don’t even think about it anymore. I did take it off on the beach in France, as it is my understanding that obvious, outwardly worn religious clothing is illegal there (this is horrible — one of the few things I deeply dislike about France). For a long time, I will confess that I worried over “what will people think if it slips around on my chest and they see it, or see the straps?”. That, my friends, is sheepishness over professing one’s religion. I really couldn’t care less anymore, in fact, if someone did see it, that would be a teachable moment.