Cut your Angles at just a RCH under 45 Degrees. Just a tiny bit. Apply Glue to both sides of the Miter. Pin them together with a Brad Nailer. Use as long of nails as possible and set 2 pins in each corner on each side. Total of 16 pins if the Wood allows. If not then just stick with 8. Make sure the Corners line up very well and all excess glue is wiped off then put them in this style of clamping device...
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Make sure you can tighten each corner individually not just the whole unit all at once. This way you can pinch a particular corner to tighten the Miter. Always use a perfectly flat surface for all work being done. This step is very important to achieve desired results. Check for Square and adjust accordingly. Set to dry and you are done.
Other tips would be to try to use one single piece of wood and keep the flow around the whole Frame. This way if there are slight differences in the wood at least the Miters are cut from very close to the same imperfections. This will match the Grain Better. You will waste a little bit of wood by cutting a Triangle out of every corner but you want to keep the same edge of the piece of wood on the outside of the Picture Frame.
Before cutting the stock for the picture frame make sure you run the whole piece through a router for the desired recess to accept the Picture, Matting, Glass and Backing Board.