In case you buy one of my hidden blade designs, I'll send you an email tutorial with pictures (I'll use my 3D models until I get one myself) showing you how to assemble it.
CAUTION: watch the size of the hidden blade. It's for a 23cm (9,055in) or bigger arm. It measures about 44,4cm (17,48in) when fully extended. Which means you get a 21,4cm (8,42in) blade.