1) Transmute your legendary on to a low level white or blue item using a Transmutation Crystal (40 gems or 2.5 gold). This will create a new account bound item with the Legendary skin and the low level item stats.
2) Place the new item in your bank.
3) Pick up the new item with the character you want to transfer the legendary to.
4) That character uses a Transmutation Splitter (300 gems or 19 gold) which gives you the 2 items back: the fully restored Legendary and the low level item. Both of these items will now be soulbound to the character that used the splitter.