Video: Double-Start (Estonian/Latvian) Cast-On
The Double-Start Cast-On is a relatively fast and easy way to cast on for 1x1 rib.
It is based on the long-tail cast-on and creates pairs of stitches as you cast on. This cast-on works best for 1x1 rib only, not other combinations of ribbing.
Because this technique creates pairs of cast-on stitches plus the slipknot at the beginning, it will result in an odd number of stitches if you repeat the technique for the whole cast-on.
If you need an even number of stitches, add one more long-tail cast-on stitch at the end.