I designed and made this for a friend and all the materials were either found or given to me. The main parts of it are the head and foot boards of a single bed (approx 95cm wide) and single bed slats to give the seat more comfort.
All the other pieces are easily obtainable and they are standard pieces of wood, screws and some wood glue. You'll be guided through the details as this process goes on.
Step 2: The Main Part of the Frame
As you can see, I sawed the foot board in half as these pieces were used for the sides of it. You should find screw holes in it from the original design which you can use to fix the pieces together. In the second photo, I used long screws (about 8cm) to make sure it holds well and there are two on each side.
Then, for the seat area you need to make a base which is also screwed to the outer parts. You can measure how long they'll need to be and you'll need two long pieces (mine were 91cm) and two short (about 50cm). Three of them can be screwed against the outer frame and the last one, which doesn't have a closed side to fix to can be secured by going through the side parts into the ends of the front piece, with long screws to made it stronger. Later, I fixed another plank under this to prevent it bending when sat on. See the later steps.
Step 3: Making the Seat
The first photo shows how the base of the seat should look - quite simple to understand.
The next part is fitting the slats. The slats that I had were shorter than the width of the seat so I needed to make some pieces that would bridge the gaps. For this, I fixed two pieces of wood together, using screws on both sides (upper and lower) to make sure they wouldn't come apart when it's sat on. They also have a bolt through the middle just to make sure (the 2nd photo here is the piece upside down). You can see the lower piece sticks out from the upper piece. This is where the slats will rest on the seat so the distance needed will be just 1cm longer than the slats so that they have room to move when they bend and straighten.
These pieces are then screwed into each end part of the frame as shown but in this third photo, you can see screws in the two ends but it will also need two screws in the piece that rest on the piece of wood that is fixed to the arm-rest part as well. This is because the weight of the person sitting will push down on the one side of this piece and detach it. Basically, put screws in everything that joins to something else!
Step 4: Setting the Slats
When putting slats in, you need to make sure they aren't fixed as they have to be able to bend. I found out some off-cut pieces of wood to place as fixed separators. Depending on the wood you use, you can fix them with wood glue if it's all natural wood. If not, then set them as in the photos and either nail or screw them down as I had to do because the glue didn't work on MDF.
As you can see, there are small gaps between the blocks and slats so that no damage is going to occur. Another way of keeping the slats in place is to get two strips of canvas or something similar and lay them over the blocks and slat-ends and pin it down to the blocks. This will just give extra security that the slats won't come out. I didn't go with this as I felt there wasn't the need once the cushion is resting on top of it all.
Step 5: Finishing Off
So here you can see how all the slats rest (make sure they are this way up as they need to press down when you sit on them, it wouldn't work if you had them upside down!)
I cut a plank to fix under the front beam so that it wouldn't bend when sat on. Again, if it's natural wood, you can glue it with wood glue or screw through the sides of the arm rest parts.
The last stage was to find a cushion to fit, which I did from a chair that someone had thrown out. My friend stripped the cover and made her own. In this case it's not fully the right shape but it's good enough for her. It still looks a bit hard to relax on but my friend prefers it like this as it better for her back. You could simply get some more cushions to put across the back if you wish.
I find it is quite comfortable and just about wide enough for two people to sit on. You could of course make a wider one if you can get hold of a double bed to use.