There are a couple of different ways you can cut the plastic so it covers the bed. One way is to cut the plastic so it covers the mattress from the bottom of the pillows to the end of the mattress and then tuck it under the mattress. You can also cut the plastic so it wraps all the way around the entire mattress. Of course you'll also have to take into consideration the size of the mattress-twin,full,queen,or king. After you figure out how you want to layer the plastic over the bed place it directly on the mattress pad covering the box spring,cover the plastic with the extra mattress pad,then cover the extra mattress pad with the fitted top sheet as mentioned above. When you're done you'll have the plastic covered with both the fitted top sheet and the fitted mattress pad.
As mentioned in my previous post on this topic,the child care author Vicki Lansky in her book "101 Parenting Tips" talked about using plastic garbage bags over the bed to protect the child's mattress. Another book on bedwetting mentioned that you can use any type of plastic to protect your child's bed from bedwetting. Also one book on asthma talked about using this type of plastic on the bed to protect the asthma sufferer from allergens that might aggravate them.With that in mind,it's not much of a stretch to use plastic from the hardware store to protect the child or teenager's bed. I had a friend who used this type of plastic on her daughter's bed when she wet the bed and it seemed to work out very well.
Another advantage of using this type of plastic is that you can cut off several sheets to use-while one sheet is drying on the clothesline,you can cover the bed with another piece of plastic. Additionally, having a roll of plastic is very handy. The plastic can be used as a drop cloth,to cover outside materials such as patio furniture,firewood,mulch,lumber,and as an air conditioner cover; for remodeling work; as a vapor barrier for insulation; for concrete work; yard work; as well as many other household chores,so even if the child or teenager can't get used to the crinkling sound the plastic makes,you can still use it for other purposes.
Regarding the crackling sound the plastic makes,the mattress pad and the fitted top sheet should make it more pleasant for the child or teenager to sleep on. That being said if they can't adjust to it I wouldn't push the issue. You can always try your luck with the vinyl mattress covers or you can use a plastic shower curtain. Also if the child or teenager has frequent visitors I would have them take the plastic off before their friends come over because if the friends sit on the bed they'll notice the crinkling sound which would be embarrassing.
Finally,keep in mind that even though the bed is wrapped in plastic,the child or teenager should sleep in diapers whether pin-on cloth diapers covered with plastic pants or disposable tape tab diapers. The plastic sheeting is not meant to be used as the primary form of protection but as a fail safe in case the diapers leak. I'm a firm believer in having the child or teenager wear some form of protection to bed to keep them comfortable,to maintain hygiene,as well as to cut down or eliminate wet laundry. Please see my blog "Tips on Managing Bedwetting" (www.tipsonmanagingbedwetting.blogspot.com) for advice about the different types and brands of pin-on cloth diapers,plastic pants,and disposable diapers with tape tabs available for older children and teenagers that wet the bed. If the child or teenager does leak through their diapers onto the plastic sheet you'll have to wash the fitted sheet and mattress pad and wipe the plastic off. I would then put the plastic sheet on a clothesline to air out. I hope this suggestion helps. I would be interested in hearing from parents who might experiment with this idea to see if it worked out,so feel free to reply to this article if you have any thoughts on the matter or questions.
http://www.articlesbase.com/parenting-articles/using-hardware-store-plastic-sheeting-as-a-bedwetting-sheet-5093254.html