Grasp the harnesses at the shoulders and pull upwards first. This ensures that the lower part of the harness is snug. Then, using the adjuster that comes with your car seat, remove all slack until the entire harness is snug.
A snug strap should not allow any slack. It lies in a relatively straight line without sagging. It does not press on the child’s flesh or push the child’s body into an unnatural position.
To test for tightness, try to pinch the flat part of the webbing between your thumb and forefinger. If the harness is snug, your fingers will slip off the harnesses without any remaining between your fingers. This is called the “pinch” test.