It's the pattern I used for the 1:12 red sofa in the little girls' house (below) and the 1:16 off white linen one in the 1920s house (second photo below) as well as the 1:16 brown check armchair (bottom photo). This armchair started out covered in the same off white linen as the sofa but when I needed a more masculine look for the study I pulled it apart and recovered it.
You can see that the off white sofa has a bit more padding - you can play around quite a lot with the final look by adjusting the amount of padding you use.
You could also change the look by altering the height of the back and squaring it off at the corners. Also it's flexible enough to bend the top of the back over to create bit of a 'rolled' look as well as adjusting the way the arms turn back - or not.
Changing the feet will also change the look, on all of these I've used round beads or pins with large coloured heads (map pins for example) but for a more modern look you can cut square feet from scrap wood and colour them to suit using paint, stain or marker pens.
The squares on this grid (my cutting mat) are 1 cm - draw up a grid on paper, you should then be able to draw your own basic shape copying the dimensions of this outline. (You can see that the back of this fabric cover isn't entirely straight and the shape of each end doesn't exactly match either - not one of my better sewing days but because this was for the kids' house I didn't redo it and the faults aren't noticeable in the finished product.)
You can shorten or lengthen the back to make it longer if you want a big 3-seater or shorten it for an armchair.
Cut 2 shapes from fabric for the back and sides cover and 1 of cardstock to provide some rigidity and 2* of thin foam. The cardstock and foam is cut to this size but remember to add seam allowance when you cut out the cover - including at the bottom for turning in!!
*If you want more padding in the edge at the top of the back and arms add an extra 1 cm to the edges on 1 piece of foam and before inserting into the cover fold this excess over to the back of the cardstock and glue into place.
On the cardstock insert lightly score the fold lines for the arms to bend forward and then fold to create a crease. That will make it much easier to wrap it around the seat block later on. Lay the card onto the foam and cut two foam pieces the same sizes (*or make one a little bigger on the as referred to above). You can also use other padding, quilt wadding for example. If you want extra padding on the front of the back and sides only take it to the top of the seat or it will be too bulky to wrap the back and sides around the seat block.
To construct the seat you can use a base of balsa or foamcore board. In this case I used two layers of foamcore to get sufficient height. Top with foam or wadding.
To cover the seat block you just wrap it up like a parcel gluing it around the sides, overlap a little under the bottom. I usually run my iron over the bottom edges where they fold under, for a nice clean line. (If you've used foamcore for the base be careful of the heat setting on your iron - I don't know if it would be inclined to melt if the iron is too hot.)
Put glue on the sides and back of the seat block, wrap the completed back and sides piece around it tightly and secure with a wide rubber band while the glue dries. All you have to do now is add the feet and your sofa (or armchair) is complete! (You can also add braid tim around the bottom if it's an older era sofa.)
Have fun with it!!
This pattern is not my own design, it's from The New Dolls' House Do-It-Yourself Book by Venus and Martin Dodge. This is the first dollhouse 'how-to' book I bought way back in the 80s. I really love this book because all the patterns are simple and the instructions easy to follow. I used it for all the furniture in my 1920s 1:16 scale house. You can see all the posts for that project here.
The first edition of this book was all in 1:16 scale but the second edition (published 1993) has patterns for houses, furniture and accessories in 1:12 and 1:16 scale. It would be easy to adapt the 1:12 patterns for 1:24 by simply photocopying at 50%.