Here's a common scenario. Clips are done up and...

They're loose. We tighten them now...

And they pass the one finger test (can fit one finger underneath, but not two stacked on top of each other)

But when the slack from the lap portion of the harness is pulled tight, look at all the slack that remains! These are the steps to take to ensure a tight fit!

Strap your child in

Straps are very loose

Pull chest clip down to crotch buckle

And pull the straps UP in order to remove the slack from the lap portion

Nice and snug on the lap!

Now, with the slack at the shoulders, tighten as per your manual

Looks pretty good...

But re-check by repeating the steps. First, pull chest clip back down to crotch buckle

And pull it up again, removing lap slack. As you can see, there is still slack there! Not tight enough yet!

So tighten again to remove that slack (slack can only be removed when it's at the shoulders, ABOVE the chest clip)

Now the lap portion is snug and so is the chest and should portion! One finger should fit underneath...

But not two stacked in one another. Chest clip should be positioned so that the top of it is in line with the armpits

Hooray!! I'm ready to go for a drive!!

Keep your child rear facing as long as the seat allows! It could save their life!