As promised I'm now going to show you one of the methods by which you can cast off on a knitting spool. Again this is how I do it; you may find different methods of casting on and off by searching the web. The important thing is that you experiment to find the one that suits you best.
So, you've knitted your tube or i-cord and are now ready to cast off, this is done pretty much as when using knitting needles.
With the hook you used to knit your tube pick your first loop and transfer it to the next peg on the left.
You now have 2 loops on one peg.
Now pull the bottom loop up and over the top loop and the peg like so.
Repeat by transfering this loop to the next peg and by pulling the bottom loop up and over the top loop and peg as before.
Carry on in this way until you have 2 loops remaining on the last peg. Again, pull the bottom loop up and over so you have one loop left and carefully remove this loop from the knitting spool
Cut the yarn (not too short!) and pull through the last loop as shown below.
Pull tightly to close the tube and you're done.