While it is customary for persons who are wearing a small scapular as compared to a complete religious habit to wear their scapulars under their outer clothes, this is not required. One may wear a small scapular over one’s outer clothes just as one may similarly wear a crucifix or any other religious symbol.
I do not recommend washing a scapular in a washing machine as it may become tangled under the agitator. Instead I put it in a clean jar large enough to completely submerge the scapular when the jar is about 3/4 full of cold or lukewarm water. (A baby food jar is probably too small, 16 oz jar is a good size, a regular spaghetti sauce jar will work but is larger than necessary unless you have a really big scapular.)
Add cold cold or lukewarm water until the jar is about 3/4 full, plus one or two drops of color safe laundry detergent. Do not use bleach or hot water unless you have an all-white scapular. Tighten the cover on the jar and let it soak for several minutes before shaking the jar vigorously for about a minute. Examine the water in the jar. If it is very dirty, discard the water, add new water and detergent and repeat.
If the wash water is only a little dirty after shaking the jar (there will always be a little dirt or lint in the water after shaking) then rinse the scapular in clean cold or lukewarm water. Air dry. To prevent tangling, I do not recommend washing multiple scapulars together.