Typically people teach the word, "stay" to mean, "Do not change position or move until I tell you to do something else or release you from this instruction." In other words, if you tell a cat to stay, while sitting, you expect the cat to freeze in a sitting posture not to lie down. If you tell a cat to lie-down and stay, you expect the cat to hold the position not rollover, get up or move in any other way.

If you want your cat to remain in a general area on the floor or to pause before going out of a door, then resist the urge to use the word, "Stay" as it will confuse the cat if you use the word, "stay" to mean freeze one time and then are lax at another time. Instead, consider using the instruction, "Wait," to mean pause here for a moment.

Any time you see your cat relaxed and laying comfortably around the house, use a hand signal to mean stay, say, "stay" and praise the cat. Of course the cat was going to stay anyway. Then stroke the cat and say, "okay" to signal the stay is done or call your cat to you.  Make sure you use a calm voice when practicing stay in this way so your cat does not immediately get up in response to your attention. The goal is to introduce the exercise of "stay" as a positive.   

Another step to teaching stay is to first teach your cat to sit on cue. After the cat knows to sit, use a stay hand signal and word to delay giving the treat. If your cat stays for just seconds, then deliver the treat while the cat is staying. Use a second treat to call the cat from the stay.