Never attach a leash directly to the collar. With them being so agile, and prone to rapid acceleration, they could cause themselves severe damage. Instead, use a harness. With added control and stability, the risk of strangulation or injury is removed or at least, reduced. Try the harness indoors first though to get them adjusted to using it. Do not force it though. An unhappy cat outdoors could very quickly become an unhappy cat indoors and then ruin what was once the perfect pet-owner relationship.