Some pets, especially cats, may feel scared the first time you put the harness on them. How can you get them used to walk in the harness? Start by putting the harness on your cat (or dog) for short sessions, about 2 minutes at a time. Associate the harness with enjoyable things, such as food, games, or prizes. Positive reinforcement also works great.
Don't leave your pet unsupervised with the harness on to start with. Be patient as your cat or dog gets used to it. Then you can start to take it outside with the harness. Familiar situations like the garden or patio would be a good start.