You need photo
editing software that supports layers and blend modes, like Photoshop or GIMP. If
you want to create, say, a tourist guide, then you place a photo of the sight
the code is supposed to link to on layer 1. Put the qr-code itself on layer 2
and apply the “negative multiply” blend mode to the layer. Check with you
mobile phone if the code works (might not work, due problems in contrast). Export
and done.
See also: How to create a mobile city guide