First train stay for a few seconds close to you in a quiet room using treats to reward staying.
Gradually step farther away, treating and releasing from distance, building up to other rooms.
Increase Distance Slowly
Slowly increase length of stay from seconds to minutes, randomly rewarding with treats.
Have dog stay while you shuffle feet or talk, rewarding calmness amidst mild distractions.
Add Low-Level Distractions
Practice stay command in variety of environments like backyard, on walks, pet store, etc.
Mix Up Locations
Pair verbal stay cue with hand signal like open palm held out in front.
Use a Hand Signal
Frequently reward small successes in early stages so dog understands desired behavior.
Reward Often At First