Remove the screws and unthread the straps from their holes
Thread the straps through the tabs on the back of the saddle, from the top. Make sure that the holster curves towards the back (see image 2), this gives the holster its shape and prevents the U-Lock from hitting the frame.
Thread the straps back through the holes. The loops holding the holster to the tabs must be tight.
Anchor the straps to the frame of the bike, on the rear brake bridge, above the rear brake. The straps should be quite tight to keep the holster in place. Trim any excess.
To keep the screws from coming loose, add a drop of lock-tight, nailpolish or glue on the threads before you screw it together.