Once you get the idea of the miters down, it gets easier but it does take repeat work. Trying something like Attic Window blocks can help. I'm pretty good with the process by now, but I like to use a 1/2" seam. Since I do my own design/cutting instructions I have allowances for that that published patterns will not.
The angles can make things easier or harder. Make sure that what you are sewing onto is square, and that your angles are cut correctly. If everything should work but isn't check the angles.
If you use a "quilting" foot on your machine, the sides should both be 1/4" and there should be a 1/4" mark on the foot to help stop. I do like to speed sew, so will often go off that first edge and remove the stitches in the 1/4" zone. To alert me to the 1/4" mark I put a pin there, higher to the edge than I usually pin, that is under the path of the foot. About 1/4-1/2" inch before that I have my warning pin.