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Start by measuring the linear footage of the space where the chair rail would be installed.  To cover any mistakes, we recommend you add 10%.

Then, choose the style and finish of chair railing you prefer.

Try to buy lengths so you can eliminate as many joins as possible.

Before you install the chair railing, you want to clear coat, stain, or paint it on both sides, allowing it to dry overnight. If you are using a clear coat, sand down the wood and collect the dust. Then when you make a join you can mix the dust with a glue that dries clear and use it to fill the gap. When it dries you can sand it smooth and apply your clear varnish, or stain followed by varnish.

Here is where even the youngest family member can join in. Move the back of a chair against the wall, and let the child draw (with pencil) along the back of the chair. Move the chair a little and repeat, until all walls are marked where you are going to place your railing. 

Cut the first piece of chair railing to the appropriate length, using a 90-degree cut, going into the corner.  While you might be tempted to use a miter cut for the inside corners, we suggest you avoid this, which can cause gaps and misalignment.

Now with a miter box and fine-toothed backsaw or miter saw, cut so the profile of the molding is revealed. Don't be tempted to work without a miter box if you are a beginner. It is a hundred times easier to use a miter box than to work the way a professional carpenter used to work.

Next place one end of the moulding on paper and draw around it on card. Cut out the card to use as a template. Place your template on the back of the next piece of railing with a straight edge against the cut end of the rail and the fancy edge facing along the length of rail. Draw round the fancy edge to give you a line for cutting. If you haven't done this before, be aware that you have to make the cut fairly vertical to the back of the rail or there will be gaps. This isn't a job for the youngest family member. 

With a coping saw, cut the second piece in the same shape of the profile of the molding.  The goal is to cut it so it can butt up against the end of the first piece firmly. A fret saw would also do the job. If some bits jut out too much you can use a rat-tailed file or triangular file as the case may be to ease the exess back to a perfect fit.

Place the chair railing upside down in the miter box, cutting off excess wood or material from the backside using a coping saw. Remember to take into account the bit you have cut off from the length of the rail when you are measuring the length of the next rail section.

With a utility knife, remove any excess material missed with the same but be careful not to expose the molding’s face.

Using 6d finishing nails, attach the chair railing to the wall studs.  In addition, counter sink the nails, fill them with wood filler, and then cover with clear coating, finish, or paint to match the rest of the railing.

Keep in mind that sometimes, you might need to pre-drill the nail holes so the wood does not split, especially when working with hardwood or thick railing pieces.