As with any design project, take some time to think about who/what you are designing for.

For example:

1) On a walk, if Toshi starts pulling on the leash, which in turn pulls on his collar, he is prone to doing what's called a reverse sneeze. He begins to wheeze and hiccup, I can't imagine it's very comfortable for him, so lets avoid touching his neck.

2) After a car ride Toshi being a small dog with a collar and leash on, will jump out of the car as soon as the door opens, not realizing that there isn't enough slack in the leash to get to the ground safely. The resulting scene scares the heck out of me as it looks like he's trying to hang himself. So let's make sure if he falls with this on, he won't get injured.

Next make some sketches while considering your use cases. They don't have to be amazing, all they need to do is explore potential ways to solve the problem!

Try making a few paper cutouts of your design, you'll start to see what's going to work, and what definitely won't, all within a few minutes. Here quality doesn't matter, in fact its almost discouraged! Fail fast, fail early, and fail often!