[djmcmath]

968rz has nailed it.

The old putty (or, in my case, carefully manufactured hand-made wooden trip pieces) come out with a chisel. Carefully. Watch for the glazier's points, those little bits of metal that hold the old trim/putty in place. If it's putty, there'll be no points, and heat (see below) will make it softer.

The new pane of glass may need to be custom-modified. In my case, the pane was square, but the hole it needed to fit into was not. Old houses, you know? We found that a belt sander was an excellent way to make _small_ modifications the shape of a piece of glass.

The new putty needs to be rolled in your hands until pretty warm, then carefully pressed into place. Heat makes the putty soft, and as it cools it'll harden. So do this before it gets genuinely cold outside.

Good luck.

Dan