Double treble crochet (US) / triple treble crochet (UK) are done similar to the treble crochet. But they are fluffier and lighter.
First we crochet a chain of chains.
Get yarn three times over the hook,
Go with the hook into the 5th stitch of the chain,
Now you have five loops on the hook.
Get the yarn over and pull it through two loops.
Now you have four loops on the hook.
Get the yarn over the hook and pull it again through two loops.
Now you have only three loops on the hook.
Get once more the yarn over the hook and pull it through the two first loops.
Now you have two loops on the hook. Get again the yarn over the hook
And pull it through two loops.
Done!