They're rectangular black plastic clips that attach to the belt area of the seat belt. When you get in, you slip the chest part of the seat belt under the sticky-uppy bit (technical term) of the clip.

I think this is where I got mine. Obviously, that second picture is not how they actually work, but it shows you how they're rigged. In practice, the belt comes down at an angle sufficient to meet the clip rather than angling all the way over to the buckle.

They're not perfect, but they do make the belt more comfortable to use.