If you have a chop/miter saw that would be ideal. Assuming you are not limited to the amount of wood available, make a 90 degree cut that represents the short end before the angle. Next, make another cut at 90 degrees that represents the long cut on the second workpiece. Then, mark the 20 degree angle, clamp them both down, and make the 20 degree cut thru both pieces. If extreme accuracy is needed, remember that the width of the sawblade should be taken into account. This allows you to ignore the stop block.
I hope this makes sense.
If the 20 degree angle is important to the build, you can use geometry or trig to figure out the angle based on the dimensions in the drawing.