Hi Susianna, glad to see you've discovered the joys of tubular style double-knitting! You'll need an extra needle for doing an open bind-off with this style of knitting - a cable needle is fine or just another needle, preferrably quite short. Your pattern for bind-off would read: purl 1/knit 1 if that's your preference, slip 1 to your spare needle, p1/k1, pass first worked stitch over the one you just worked, slip 1 to your spare needle, p1/k1, pass the first stitch on your main needle over this one. Continue this way until you just have the slipped stitches to work and bind these off in the usual way.
If you have the right number of stitches cast on and have worked the right length you can use your pattern to make phone covers or any other similar shape. You just need to work some rows of ribbing at the top of the cover to make it stretchy so that your phone or whatever will fit in.