Make sure the glass and channel is seated in the frame before putting the clamps on tight. I cracked the corner because the glass was not fully seated.
With the glass and channel installed in the frame and the frame sitting vertically on a piece of carpet or foam you can use a rubber mallet and a piece of wood to seat the glass. Light raps along the edge of the glass should work. Keep the face of the mallet flat to the edge of the glass. These are suggestions only and I take no responsibility for broken glass etc.
Or you could just take it to a glass shop.