First you need to make sure your parts are the same length before you try to glue them, height the same, width the same.
To see if the door is square measure across the diagonal from corner to corner. They should be the same measurement. if they are not then adjust your clamps until they match.
You can also check the angle between 2 sides to see if it is 90. Just remember 3,4,5.
Measure along the shortest side and mark a measurement that you can divide buy 3 equally. Say it is 12". 12/3 = 4. Now multiply the other 2 sides buy 4, you get 4x4 = 16, and 4x5 = 20. Measure along the other leg of the door and make a mark at 16". The measurement between the 2 points you marked should be 20" if the angle is 90 degrees. If it less than 20" then the angle is less than 90 degrees, if it is greater than 20" then the angle is larger than 90 degrees. 3,4,5 .... 6,8,10 .... 9,12,15 .... 12,16,20 .... does not have to be even but it helps with the math, it could be 7.5,10,12.5, just as long as the ratio is the same 3,4,5.
You can adjust your clamps to correct the angle.
Hope this helps,
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