If you have purchased scope-rail from here http://target-tune.in/international/products/IHP-scope-mount.html then follow this DYI
This is all what you need Yes screw and drill machine is not shown here. Just follow the picture.
Place the rail (the side marked in red) as shown in the picture:
for 4mm Tap you need 3.2mm drill
Once done, dismantle (You can use the following link) the gun. File off and wash out unnecessary material. Assemble them back. Once done, use the screw to fix the rail, check if you can cock. If not remove the screw which is located towards the back sight file off a little (say 1mm) and screw it back. You should be able to cock now.