First let me explain why you would want to build a signal box when you could just put levers by the track. Well firstly if you are running multiple trains at once and they are going on the same track and I need someone to be monitoring what train goes where secondly, why not?

1. Find the stretch of track you want to build your signal box in

2. With red wool outline the signal box shape roughly in the middle of the stretch of track ( the size of the signal box depends on the amount of points or signals or realy anything you want to control).

3. Now replace the red wool with a block of your choice and build it up by three or more layers (this is important) then ont op of that put what block the floor will be and blue walls, make sure the ones facing the track are glasses you can see.

4. Place levers making sure they are one block apart

5. Then inbetween those levers put dummy ones make sure you know which ones are real and what ones are fake.

6. Then with the levers dig under the floor and into the space that should be empty and place blocks leading the redstone down to the floor and dig two block deep trenches leading to the points (be aware rail blocks are those annoying blocks that need the red stone to go directly to them) switch the levers and test of the points or signals work. of they don't try again .

7. Then once you are confident all the points or signals work cover up all the redstone and place the floor back in the signal box, now it's decorating time you can put anything you want in it like beds furnaces chairs anything.

8. just like that you should have a working signal box