There are probably already other tutorials online for making something similar, but sometimes I find it helps to read things written in a different way, so here's my version:
Start out with 3 rectangles of fabric, one for each bow and one for the 'knot' in the middle.
My large bow piece measured 35 x 15 cm, the small bow measured 22 x 13 cm, and I ended up cutting the 'knot' down after taking this photo, so in the end it was about 4.5 x 10 cm.
Fold the large bow piece in half, right sides together, and sew down one short edge and down the long edge, then turn right sides out.
Repeat with the other bow piece and the knot piece.
Take the large bow piece, and fold the ends over so that they meet in the middle, with the finished edge on top:
By hand, sew a loose running stitch down where the two ends join...
Then pull the thread through to gather the material up into a bow shape.
Secure the end of the thread, then repeat the whole process with the smaller bow piece.
Place the two bows on top of each other and secure together with a couple of stitches - this is just to hold them together while you add the 'knot'.
Wrap the 'knot' piece around the middle of the two bows, with the two ends meeting at the back with the finished end on top. You will probably need to cut some length off the raw edge - I prefer to have too much and have to cut some off than not have enough!
Sew the ends of the knot together by hand, and that's the bow finished! Ta da!
Now all you need to do is attach some elastic. I find it's best to just tie the elastic to the right length on your head - that way you can work out just how tight you would like it (and I have a massive head, so if I gave you a measurement based on what I used, it would probably be too big for most people!).
Tie the elastic into a knot, and sew the knot onto the back of the knot on the bow. This doesn't need to be too neat (as you can see, mine definitely isn't!) because you're about to cover it up.
Cut a small piece of felt (other fabric would be fine too, just with felt you don't have to worry about edges fraying) to cover the elastic on the knot and glue on (I used Gutermann creativ HT2)
And you're done!
Here's a picture of me wearing mine at my friend's wedding this weekend.
If you make your own versions, I'd love to see them!