‘Stay’ training naturally follows once ‘sit’ or ‘lie down’ training is reliable. Instead of immediately rewarding a ‘sit’, say ‘stay’ and give the reward. Put another treat in your hand in front of your cat’s nose, say ‘stay’ again but now wait two seconds before giving the treat.
Gradually increase the interval, seconds at a time, between the request ‘stay’ and the reward. If your cat doesn’t ‘stay’ go back to a ‘sit’ and when it complies end the training session and try again when its more in the mood. Eventually you should be able to get your cat to ‘sit-stay’ or ‘lie down-stay’ for more than a minute
Cats find constant corrections annoying so avoid frequent lifting and returning you cat to where you want it. Be patient. Go back a step when needed, or after a short ‘stay’ add a ‘come’ before your cat thinks about leaving the ‘stay’. Some cats learn to ‘stay’ better on raised surfaces such as tables than they do on the floor.