These are great force multipliers.while in the store buy some thread, I usually just pick up a thread and needle repair kit they sell, it comes with multiple thread colors, not a big issue, but may help if you have to repair a button on something. Also pick up a package of clear tape, lots of usages for this, perhaps another post.
Now you are set. The alarm package contains two pieces, the alarm and contact. Both have peel and stick backs, so in the right situation will work right out of the package. But they do have problems with metal doors and door frames, so that is where the thread and tape come in.
By the way do your testing with one alarm, leave the other packages sealed. I find this the best way to insure I have the battery life for a 1 maybe 2 day gig, is to use ones I open just before using. I have reused with new batteries but normally for training folks.
Ok if you have it working right out of the package great, you are in play. For others, no problem, mount the alarm on the wall use some of your tape if you have to. Now tie a piece of thread to the contact, set it on top of the alarm, now tape the other end of the thread to the door, as the door is opened the thread will pull the contact off the alarm, and the alarm will sound.
Now I use these for fire stairs or other doors that I want to know if someone is coming through. But I also used them as room alarms for my hotel room to insure I wake up if someone tries to enter. But the uses are endless. If I want to cover a hall way leading to a room, late late at night, when no one should be walking around. I mount my alarm on one wall set the contact with the thread attached on top of it, and run the thread across the hallway and tape the thread to the other wall. Now anyone coming down the hall will set off the alarm.
But I use the thread and tape routine in many ways, to my briefcase in my room as I go out to dinner, I have used it to secure my briefcase while on long layovers in airports, when I am tired and may fall asleep. It is just a tool, and the bad guy can defeat it if they are aware of it. but it often gives you that edge you need when understaffed.
For a few bucks, everything is a dollar at the dollar tree, you have options to McGyver your security.