With miter saw, you would mark the lines on the sides and cut to the lines, inside or outside the line. Compound miter saw is better but as long as you have a miter saw table, search the images, and the post clamped down, cuts can be made safely.
You can do the same with circular saw. I think circular saw blade should stick out that much with the angle. Safest when you have a good sturdy work table, no wobble, and clamp the post down. Free hand the saw and cut to the lines. Since such posts are not planed and square, and the saw is following the imperfect lines, your cuts may not look the best. You can clean up with chisel and sanding.
Having said all that, circular saw is better. Miter saw, the post needs to be flat and straight. Circular saw can be used, example, even if the base is free in the air. You just have to hold the saw even. At full power and slow steady pace, and following the line absolutely, there is very little chance of kick back.
If you're not familiar with circular saw, and try to guide it with one hand, there's more chance of accidents. Some cuts, I would use the hand saw for the whole cut or to finish and clean with a chisel.
