This would be my process.
1. Measure the quilt top to determine the width and length. (I'm not going to go into the process of measuring in several places, but you know you have to do that.) Write down the width and length.
2. Cut the borders for the sides using the length of the quilt and attach.
3. Cut the borders for the top and bottom using the width that you originally wrote down in step 1 (not the current width) and cut those borders.
4. Now cut the cornerstones. The cornerstones should measure the unfinished width of the borders on the sides by the unfinished width of the borders on the top. If the borders are the same width all around, then the cornerstones will be square. Attach a cornerstone to each end of the top and bottom borders and attach, matching seams.
Repeat all steps for each border. The measurement in step 1 will change as you add borders. Always use the latest measurement.