Measure out the correct length for the corner post, then use a pencil and a straightedge to draw a line across the post where you need to cut it.
Lay the post down on the cutting area of your miter saw and match the pencil line with the cutting line on the miter saw. With the miter saw turned off, lower the blade down to the pencil line. Adjust the post as necessary so the edge of the blade is right on the line you drew.
Turn on the miter saw and lower the blade down to the post. Slowly move the blade through the post until the two sides separate from each other. Turn the saw off and raise the blade back up to its normal position. Your deck railing corner post is now cut and ready to be installed on your deck.
Repeat for other corner posts securing the deck railing.