I love the Turkish cast on for a seamless start to my toys or top down hats (it's primarily used as a toe up sock cast on) but most instructions call for two circular needles.
I never have two circular needles of the same size (why would I buy a second circular needle if I already had one that size?) so I've been struggling along making do with double point needles and then having to tighten it all up later. A bit of a faff for what should be a neat start to a project.

This tutorial shows the technique I unvented for this cast on with only one circular.  
In the pictures I'm using DK weight yarn and a US size 6 (4mm) needle with a 13.5" (about 34cm) long cable.  I think it would be a nightmare with a short cable circular needle but feel free to prove me wrong
1. Make a slip knot and put it on one of the needles.
2. Hold the needles together with the knot on the needle closest to you.
3. Wrap the yarn over the needles away from you and under the needles back towards you.  I'm casting on 16 stitches so I need to wrap the yarn eight times.  You always wrap the yarn half as many times as you need stitches. 
4. Hold the working yarn as though to knit.
5. Pull the needle farthest away from you all the way out so that the wraps lie on the cable.
6. Knit the wraps (in this case eight stitches) with the needle you just pulled out.
7. Pull the left needle out of the slip knot.
8. Now you need to rearrange the circular so that the unworked wraps end up on the left needle and the stitches you have just knit are on the cable.  You will probably need to put the worked stitches on the cable to have enough slack to get the wraps onto the needle.
9. Knit the remaining wraps.  Make sure to count, the last one sometimes slips off, if you find you are one short just wrap the tail of the yarn over the needle and work it.
10. You now have all 16 stitches cast on. 
11. You can knit off the circular on to double points or two circulars or do the magic loop - whatever you prefer - and you have a perfect seamless cast on.
Let me know if you find this useful or if you have any suggestions by getting in touch via the About & Contact page.

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