Always measure both directions N/S & E/W starting from the middle, make sure middle & both ends measure the same. Start binding pinning to the middle, working to ends. This keeps it from rippling. (borders too). Always square up your blocks before stitching them/rows together. And Sashing can be added the same way if using long pieces, as you would your borders/bindings.
Remember, adding a 10" border to a quilt, doubles it "inches" size if your sending it out to a long arm quilter, so you'll probably be charged double the cost of that size quilt. Borders are often added to expand quilts, create a bed side drop. Old quilts didn't have borders, and I just made a 1/2 log cabin pattern that is new, and had no borders, looks nice.
The Carpenters Square (L shaped) mentioned before is also a good tool, I have one just for me, so I don't have to find hubby's. Debbie D