I thought I’d share some of the resources I used to work through the Time of the Doctor Scarf pattern. The best book I found (ps I don’t make anything from these links, I’m just sharing.) was Extreme Double Knitting by Alasdair Post-Quinn. This book is a bible of double knitting. Everything you need to know to get started in one place.
There were also some really good videos on youtube that helped me get going.
Good luck on your double knitting adventure.