Equipment needed

Steps  

1. Measure 76 mm from  point 1 on image along the top of the second slot from the top, and mark a line down the duckbill perpendicular to the slots at point 1a. Note: it is better to go longer then the 76mm point then under it, as you will file off any extra material to fit your own rifle later. 
2. Clamp Magazine to work bench, to make cutting easier, and using hacksaw cut along this line
   
3. Using hacksaw cut along the top of the second slot up to the end of the slot.
4. Draw line on mag as marked below on image and cut along it 
   
   
5. On the top edge of the duckbill mark a point  5 mm from shoulder of mag and use straight edge to draw line to the 76 mm mark from step 1 
6. Cut along this line
7. file down magazine tab to fit your rifle and you are done.