You will need:
A selection of buttons in various sizes & shades of your chosen colour
Some pliable craft/floristry wire
Jewellery making pliers and snips
A toggle clasp
1/2 metre of thin satin ribbon
A needle and some thread to match the ribbon
Using the largest buttons first, and starting from the centre out, arrange the buttons until you find combinations you like. You can use two or even 3 buttons for each link, some may fit inside each other, while others will just layer nicely. Decrease the size of the buttons as you work towards the ends of the necklace. Once you've settled on your design, you're ready to start wiring them together.
Cut a piece of wire at least 50cm long, so you have plenty to work with. Fold the wire in half to find the centre point, and then thread on your centre button.
Then take the next buttons from the left, and thread those next to the centre button - Tuck the buttons slightly behind the centre button, so there is an overlap.
Repeat this step with the next buttons from the right of your design.
Keep adding the buttons, alternating from side to side of your design. It is possible to use buttons with a shank, rather than regular buttonholes as a top layer if you wish, provided the bottom button has a deep central 'well' to accommodate the shank.
As you pull the wire tight, the top button will then sit flush on top of the bottom one.
Continue threading your buttons on the wire, remembering to always tuck the one you are threading slightly underneath the previous button - the slight overlap helps the finished necklace to hold it's shape and rigidity better.
When you have threaded them all, bend the piece into a slight 'U' shaped curve - the front should look like this...
....and the back should look like this.
Take the ends of the wire, and form them into small loops behind the end buttons on either side.
thread the loose end of the wire back through the next button along, so it's poking back out at the front.
Then snip the end off as close as you can to the top of the button - This will prevent any sharp edges digging in to your chest while you are wearing your necklace!
Turn the piece back over so you are looking at the reverse, and find your satin ribbon. Thread the ribbon through the loop of wire at the back of the centre button.
Working from the centre outwards, wrap the ribbon over and through the wiring so that it loops across the back of the buttons, finally threading it through the loop of wire you made at the end (The ribbon should then obscure any visible wire at the edge of the button which may have been visible from the front.
The finished back view should look like this...
...and from the front it will look like this.
All you need to do now is to sew the toggle clasp into place at either end of the ribbon, once you have decided on the length of your necklace. I've always found that these work better as a shorter necklace, with the end buttons sitting just on the collarbones - Due to the fact that it is wired together as one solid piece, if you leave it much longer than this, it will tend to swing forward when you lean forward and hit you in the face! Alternatively, if you didn't want to use a clasp to fasten, you could just tie the ends of the ribbon at the back of your neck at the required length.