I found double knitting infuriatingly frustrating... but I managed to cast on and do a couple rows before deciding it was not for me.
You will cast on holding both colours of yarn. When you have finished casting on, you go back and make sure they are lined up properly. You can do this by pulling one over the other to line it up... Ill use red and black as my 2 colours.
It should always be RED BLACK RED BLACKthis can reversed to BLACK RED BLACK RED Basically you just want to make sure that the colours next to each other are opposite... so there should be no RED BLACK BLACK RED....
Once you have the cast on, you can watch amy's video and follow what she does for the general knitting. It is a shame she doesn't have the cast on method shown here...
Make sure to keep your yarn balls seperate. Things can get tangled quickly.