This guide will probably only be required in the event the front glass is removed (following the guide found here Samsung Galaxy S III Broken Front Glass Replacement) and the LCD display is cracked.
Alternatively if you want to glue the LCD display to the replacement glass using clear glue you may find it easier to do this with the display removed.
This is not to be attempted by the faint of heart and whilst you won't damage the phone you may damage the display, as such I would only recommend this in the event the display is damaged and not working.
Use the iOpener instead of the heat gun if you have one to remove the front glass, it makes life easier and massively reduces the risk of damaging the phone due to excess heat.
This guide include the steps for the removal of the front glass however SKIP steps 12-15 (the last steps of the front glass) as these detail actually reattaching the front glass which you won't be doing at this point