Step1
4x4 cedar post, single notch.
There are many types and styles of railing posts for a deck. For this example, I will cover posts that can either be treated pine 4x4's, or cedar 4x4's. Your typical railing post is a 4x4, unless you are planning on building an elaborate railing with split rails on each side of the balesters (spindles) with a cap (or hand rail) on top, and that requires a 6x6 post.
It is easy to notch both 4x4's and 6x6's. The distinctive difference between the two is this; you can notch the 4x4's with a compound miter saw w/ a slide arm, but you can only use a circular saw on a 6x6.
Most states require the top of a deck railing to be at least 36 inches in height, therefore, cut your posts (regardless of what size you are using) at 51 inches. This will make the post long enough to hold it at the bottom of the deck joist for nailing, and still have 2-4 inches above the railing. Most states also require the railing to be no more than 2-4 inches above the decking, (we will use 4 inches here), so cut two scrap pieces at 4 inches and set them aside for later.
Use the factory end of the post for your notches. Measure back 1 inch from the end and make a mark. continuing down the post, make two more marks; one at 3 3/8, and one at 3 7/8. this will give your post a square and ring look to it (see figure 1). Using the speed square, mark a line all around the post according to your measurements. You should have three lines in all.
Step2
4x4 cedar posts, double notched.
Next, set your saw at a 45 degree angle, and cut the first line you marked at one inch. Be sure to keep the blade straight, and try to cut it with one pass. If you try to start over half way through, the angles will not match up on the corners, and the blade marks will show on the wood.
Once that is done, re-set the saw to 0, and set the blade depth to 3/8". This is for the second notch. start with the top line, and cut it once, then cut the bottom line. Now make four or five cuts between the top and bottom lines. Now chip the cut wood out with a chisel. Once all of the big pieces have been removed, use the saw to clean up the notch. Do this by holding back the blade guard and sweeping back and forth from one end of the notch to the other with the saw blade running. try not to push too hard against the notch wall, as the blade with make the notch wide in that one area.
Repeat these steps for all four sides of the post.
Below is an example of a double notched 4x4 cedar post (see figure 2). For instructions on how to do that, contact me through this web site.
Step3
example of 4x4 treated stair posts.
For the example 10 x 12 deck, a set of steps will be added later, so be sure to include the 4 posts needed for the stair railing when estimating how many posts you need for your deck.
A 10 x 12 usually has nine posts; seven on top, and two for the bottom of the stairs. There will be a post in all four corners of the deck, one in the center of the front, and one in the center of a side. The seventh post is for the steps, and is usually 36 inches from another post (depending on where you want your steps to be) see figure 3. For the first one, measure out from the house 1 1/2 inches and make a mark. then measure from the end of the deck 3 inches(only if the decking is hanging over an inch and a half), and make another mark. connect the two lines with a speed square, that will give you a pattern to set your post on so you can trace out the rest of it. Drill two holes diagonally from each other in two of the corners. Then use the jig saw to cut out the post pattern. When cutting next to the house, be carefully, and go slow in the corner to avoid the angle bracket and nail heads.
Once the hole is cut out, insert the post and see if it can be plumbed. do this by holding it in the hole with a post level attached to it (the same way you leveled the posts in the ground). When it is plumb, nail it to the joist with the framing gun. Then you have to brace the sides with 2 x 10's ( or the same size of your joists). Nails the supports to the post, and the joists on each side. After the post is supported, drill two holes for the carriage bolts, each of them an 1 1/2 from the top and bottom. Your post is now set.
6x6 treated, single notched posts.
4x4 cedar post, single notch.
There are many types and styles of railing posts for a deck. For this example, I will cover posts that can either be treated pine 4x4's, or cedar 4x4's. Your typical railing post is a 4x4, unless you are planning on building an elaborate railing with split rails on each side of the balesters (spindles) with a cap (or hand rail) on top, and that requires a 6x6 post. 
4x4 cedar posts, double notched.
Next, set your saw at a 45 degree angle, and cut the first line you marked at one inch. Be sure to keep the blade straight, and try to cut it with one pass. If you try to start over half way through, the angles will not match up on the corners, and the blade marks will show on the wood.
example of 4x4 treated stair posts.
For the example 10 x 12 deck, a set of steps will be added later, so be sure to include the 4 posts needed for the stair railing when estimating how many posts you need for your deck.
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