Hello: I am a complete beginner, and have just learned the double cast on. But my cast-on stitches are sometimes very close together on the needle, and at other times over an inch apart. I assume this is due to bad releasing and tightening. Most videos explain the double cast-on very slowly and clearly, and then rush through “release and tighten.” I assume that’s because most people find release-and-tighten easy and obvious. Unfortunately I’m really confused by it. Can someone tell me exactly how to release and tighten after a double cast-on stitch? For instance, do I pull down the loop my thumb is in, and then the tail? Do I pull them at the same time? (Etc.) Apologies for such an amateur question, and I would love any advice you can give me. … Verity.
No question is too simple. We’ve all had to learn.
When you tighten up the stitch, pull down on the thumb to snug it up. Not too tight–keep practicing and you’ll get the right feel.
Also, keep your stitches close to the tip so you don’t get a stretched thread between them.
I cast on the same cast on but execute it a different way. I just went and found a video of someone doing it the way I do. It is called the thumb method. It makes exactly the same cast on as the usual one shown where you hold both yarns in the left hand.
This video is short and shows it quite well. The way she did her slip knot was interesting too, but she had taught it in a previous lesson and doesn’t explain it very well, and I couldn’t replicate what she did. 
Just put a slip knot on the needle however you prefer to do it and then follow the next steps.
As you can see, all you pull on to tighten the yarn is the yarn that was on the thumb. Practice will make the tension you give each stitch the same amount of pull and make them very even.
I know how to do it the “sling shot” method (all on the left hand) as well, but find it a little tiring to my hand to do it that way. You may still prefer to do it this way though. What I do when I do it that way is… at the point where you have the stitch on the needle you have to take your left thumb out of the loop, and then pull on the tail. To pull on that tail end I stick my left thumb behind the tail part of the yarn that is running into my palm and held there by my other fingers, and then I stretch my thumb out toward my body to tighten the stitch. Then I twist my thumb a little to bring it back into position to make the next stitch. I find that stretching of my thumb over and over to be a little tiring, but it can work well, with practice to make nice even stitches.
I recommend that you watch this video and then use either method, but you will see clearly that it is only the tail part that gets pulled to tighten the stitches. Link to video.
I do my long tail with 2 hands too. I couldn’t get it when I first began to knit, then discovered that ‘knitting’ the stitch with the right hand worked just as well. I’ve lately tried LT the ‘proper’ way, but find that I constantly have to adjust the yarn on the thumb and my tension is all over the place.
Sue, my friend in high school taught me that way and it always worked well for me so I never switched to the ‘sling shot’. The ‘sling shot’ is good to know for dong some other cast-ons that are a take off of LT, like the Norwegian twisted CO.
Thank you all, so much. Keeping my stitches toward the needle tip did help a lot, and I find the thumb method suits me much better than what I had been doing. Now to practicing … which I find very soothing. Again, many thanks! … Verity
You’re welcome.