Since it's a simple model, let's start with the air vent. If you want it nice and all though, you will have to spend a bit more time!
-First the Rectangle tool to create a square in that group.
-Then a smaller square in it so we have the border.
-I delete the face of the inner square to create the hole
-And then I Pull the border out to give it a bit of volume.
A bit of creativity is a good thing. Don't forget you can Push and Pull various objects to add a bit of volume to them, as long as they aren't obstacle characters can run into, like some pieces on the walls. Adding a bit of volume here and there where you can is a way to increase esthetics and realism.
-Then don't forget to create the group
Now we have to make the bars of the railings. To do so, try to keep precise measures. The measures should be readable at the bottom of the window, but if it's not the case contact me and I'll you give the informations.
-With the Pencil tool, I mark the spots where I'll start to draw the bars. The height of the vent being 2,77m, so with 10 bars the spacing has to be 0,27m. I'll make triangular bars like in a real air vent. When all measures are taken and marked, I delete the rectangle face that got back there when marking the spots.
-Generally those bars are slanted, so I draw my lines a little higher than on the original. I draw them on the red axis with the thickness that fits; in this case I went for 0,25m.
-To make the bars oblique, I draw a vertical line (blue axis) of the desired height. Here my line will be 0,08m and then I'll go up another 0,08m for the next line, that will be the height of these objects.
-I connect those points and the starting point together, I delete the red and blue axes with the Eraser tool; we end up with our a triangle.
Now we want that triangle to be a 3D model, so once again we resort to the Push-Pull tool.
-Moving slightly away will form a new line. Drag the arete and pull it out, that will elongate the object towards the opposite face.
-Create a group with that piece.
-Now that the first bar is done, we can just copy-paste it (Ctrl+C, Ctrl+V) to duplicate it. For each of them, I place the front edge of the bar onto the marks that were drawn earlier, for precision!
But watch out if you paste the model and move the camera, the piece will disappear. If that happens, you'll need to Ctrl+V it again. And if it is misplaced, use the Déplacer/Move tool to put it in the correct spot.
-Since I'm aiming for something faithful to the original, I'll add the 2 vertical bars too. I repeat the process but this time for 2 Pulled Rectangles going across the railings.
-When an object is created and if it's meant to have one same texture, don't forget to create a group for it, that'll make texturing easier later on.
Here's a rendering of our scene so you can see where we're currently at!