The insert needs to overlap the opening it goes into by double your seam allowance. If your seam allowance is 1/2" the insert needs to be 1" bigger all the way around. If your seam allowance is 3/8" the insert needs to be 3/4" bigger all the way around.
Another trick of the trade is to make the inset panel an inch wider than it should be, center the inset panel in the middle of the piece above the insert, sew to the corner where it turns 90 degrees, turn the corner and follow down the insert the same distance from the outside edge of the insert. It should be 1/2" in from the edge of the insert, but it may be 3/8" or 5/8". Whatever it is, follow down the edge of the insert that width. Then do the same thing sewing to the other corner, make the second 90 degree turn and follow down the same way.
The point is that you can always make the insert smaller, but you're screwed if it needs to be bigger.
No one lives forever, the trick is creating something that will.
Last edited by DanTwoLakes; 02-11-2011 at 02:28 PM.