Position the 4-by-4-inch post on a sturdy work surface where it will be well-supported during cutting. Consider having an assistant hold the post for safety.
Mark the position on the post to be cut. Place a carpenter's square on the post so that one side of the square runs along the length of the post and the right angle is positioned at the point to be cut. Draw a line along the right angle of the square as a cutting guide.
Put on a pair of safety glasses. Plug the circular saw into an electrical outlet.
Place the saw on the post. Hold the circular saw with one hand and the deck post with the other. Align the front of the blade with the marked line on the post. Back the blade away from the edge of the post slightly.
Pull the trigger to start the saw blade turning. Guide the spinning saw blade through the post, following the line. Allow the saw to stop -- the blade cannot go completely through the 4-b-4-inch post. Set the saw aside.
Turn the post over. Align the saw blade with the groove from the first cut. Repeat the steps to make a second cut, which will finish cutting through the 4-by-4-inch post.