Here are the two most important tips I can think of:
-First off you want two panes of glass, and they need to be two different thicknesses (so that they vibrate at two different frequencies which will decouple them acoustically). Go as thick as you can afford (mine are 1/2" and 3/4"). The pane in your recording room needs to be angled down slightly (the top of the pane should be at least 3" closer to you than the bottom of the pane). It doesn't hurt to angle the control room side pane either, but I find the sound room side to be the most important.
-Once you find out the exact dimensions of the pieces that you need, call EVERY glass place in your area for a quote. After calling 5 or 6 glass places I found the average price for the two pieces I needed was about $400. So I got a list of EVERY glass dealer in town and made my drummer call them all and get quotes. In the end I got both of the pieces I needed for only $160.
-Lance