1 - Neck
You should be able to fit at least two fingers vertically between the top of your dog’s neck and the harness for a medium-sized dog (sizes 4-6). A bit less for smaller dogs, more for bigger dogs.
Fingers on top of each other, between your dog’s neck and the harness.
2 - Chest
For medium-sized dogs, you should be able to fit at least two fingers vertically between your dog’s chest and the harness. Adjust on both sides, as even as possible. After initial adjustment for your dog, lock the chest straps by simply threading the end of the strap back through the slip lock.
3 - Armpits
Make sure there is room for your dog to move freely.
If you find it hard to take the harness on/off, twist it sideways when pulling it over your dog’s head.