I have a large wardrobe at home that we want to use as a bookcase, but it is missing the wooden shelves. The thing is that the wardrobe is quite wide, so the shelves could get to bear a lot of weight if they are completely filled with books. So as it would be necessary to reinforce the shelves I put in, I thought it would be better to make the wooden shelves already reinforced.
To make the shelves for the books I bought tongue and groove boards 22 millimeters thick and 14 centimeters wide.
I originally planned to make a single shelf, but then I decided to make two. And I used two boards to make each one. And to reinforce the wooden shelves I decided to use a 38-millimeter square section wood strip (which I already had in the workshop) between the two boards, and another 38-millimeter high and 22-millimeter thick strip to make the front of the shelves. Thus, most of the work consisted of routing the tongue and the groove in the wood strips in order to joint them with the tongue and groove boards.
To mill the tongue and the groove in the edges of the reinforcing strips, I used my DIY router table with a router bit that is thinner than the tongue and the groove of the boards. Then, it was a matter of adjusting the hight of the slot cutter router bit to mill the tongue and the groove to the correct size and in the correct position to fit with the tongue and groove of the boards. To make all the adjustments and to route the workpieces, I always put the boards and strips with what will be the top surface of the shelf (which will be the surface on which the books will be placed on the shelves) against the surface of the router table.
I first matched the bottom cutting edge of the slot cutter router bit  with the bottom surface of the groove of the board . And I routed all the wood strips. Then I adjusted the upper cutting edge of the router bit  to the upper surface of the groove  and finished milling the groove in all the strips. And yes, I managed to correctly adjust the fence of the router table and the height of the slot cutter router bit. So now have already made all the grooves with the correct dimensions and in the correct position.
To mill the tongue on the edge of the wood strips I started by routing a small rabbet in the lower corner . This is to avoid leaving that corner unmilled. Now I have to adjust the upper cutting edge of the router bit with the lower surface of the tongue of the board . And I route the strips .
Then I have to adjust the lower edge of the router bit with the upper surface of the tongue of the board . And I finish cutting the tongue in the edge of the wood trips . All I have left to do is to mill a small rabbet above the grooves in the boards  so that all the tongue and groove joints will fit.
This way I have strips that will fit between the tongue and groove boards in a way that the top surface of the shelves will be flat (in this photo the surface that is against the table, which will be the top surface of the wooden shelf). These square section wood strips, being thicker than the tongue and groove boards, will serve to reinforce these wooden shelves and so support the weight of the books.
To glue all the pieces of each shelf I use my long woodworking clamps made of wood. I use some pieces of newspaper so that the carpenter’s glue doesn’t stick to wood clamps. A little help from my cheap clamps and I can clamp with confidence that the shelf will be completely flat once the glue is dry. Well, I also have to glue a few small pieces of lath under both sides of the shelves (you can watch them in the next picture). These are to even out the surface that will rest on the supports in the wardrobe.
Once all the glue is dry I have to adjust the sides of the wooden shelves with the disk saw. With the help of my circular saw jig it is easy to remove just a couple of millimeters, leaving the sides of the shelves perfect.
With the orbital sander I go over all the surfaces, getting rid of the very small steps that I had left in some of the joints between the wooden boards and the square section strips. I prepare the surfaces of the shelves by sanding progressively with finer sandpaper and I apply a protective stain.
When applying the dye it is difficult not to leave brush marks, so the best solution I have is to apply a generous coat and then remove the excess of protective dye with newspaper. It takes several sheets of newspaper, gradually removing the excess dye, but eventually I get a smooth dyed surface without any brush marks. I think I got a nice color for the bookcase.
This was not the case, but it can happen that a sandpaper-like surface remains, but it can be smoothed with a soft sanding, between coats of stain, with very fine sandpaper. If desired, apply another coat of dye, but don’t forget to remove the excess. After the second sanding the surface is usually completely smooth even if more stain is applied on it.