First, adjust the front and back straps until the harness is nice and snugly fitting - you want to be able to fit one fingertip only between your cat and their harness when they are in neutral standing position. Make sure that the back straps pass behind (not over) your cat's front elbows.
Achieving a good snug fit is important for escape proofing but also comfiest for your cat (the less they can feel the harness slide across their fur as they move, the more relaxed they'll be!).
Remember to adjust both left and right sides of the straps equally so that the harness stays centered on your cat's body.
The main buckle
The ideal position for the main buckle is directly over your cat's shoulderblades, as this creates a perfect balance between the front and back straps on your cat's harness.
It can sometimes look as though the buckle is uncomfy for your cat in this position, but don't worry, it's perfectly comfortable for them! The buckle is specially designed for kitty comfort - with a smooth surface to cause a minimal amount of friction against their fur, so that they can forget all about their harness and get on with the important business of exploring!
My cat has been using another style of harness, and seems a little bit uncomfy or freaked out in their new Gatsby harness
If your cat has been using a different style of harness in the past, you may find that they require a little bit of time to settle into wearing their new Gatsby style of harness, and get used to the new sensations of where the harness touches their body. You might like to take a look at our Harness Introduction Guide over on the blog - it's written for harness first-timers but the process is the exact same for kitties who are switching over from a different style of harness!
My cat is a little bit chunky and I'm worried that the back straps are tight under their front legs
If your cat is on the chunky side you may benefit from changing the angle at which the back straps come down behind their elbows.
To achieve this, you can experiment with letting the harness' front straps out a little longer, and taking the back straps in a little tighter. The end result will be that the main buckle moves further down your cat's back (towards their tail) and creates a less sharp angle on the back strap behind their elbows.
Just remember to check that once you have made your adjustments you are able to achieve a good snug fit overall (that allows you to slide one fingertip between your cat's harness and their body when it is on).
Try to be conservative with your adjustments - while it's OK to move the buckle so it sits just behind your cat's shoulderblades, pushing it much further back on your cat's body can prevent the harness' escape-proof design from working effectively.