Look carefully and you will see that one end of the "T-piece" (the bit that goes into the buckle end of the strap) has a screwdriver slot in it. Simply unscrew, remove, wriggle the remaining bit into your strap and then replace the part you removed.
Be careful. Some of these deployants have two removeable parts. They can be a bugger to assemble again - yfor these, you'll need two screwdrivers, three hands and a lively sense of humour.
Having fitted it, you will probably find that the deployant is bloody uncomfortable and you'll will remove it and revert to a tang buckle as soon as possible.