I use a jig like the one shown by Rick and cut pieces on the left and right side. After purchasing my Kreg miter gauge, I tend to use it as the cuts come out accurately. I usually cut them intentionally about 1/4-3/8 inch longer than needed. If I need to adjust the angles a bit, the extra length allows me several adjustments. I check the length of pieces on opposite sides by standing the sharp corner on my saw table and putting them back to back to see how close the tips come to each other. You can easily see differences as fine as 1/64". It is critical to proper fit up that opposite sides be exactly the same length. Once I verify that I have the angles right, I can trim the pieces to the correct final length.