Assuming you are right handed.....
1. Hold the roller in your left hand (it may be an idea to unwind as much fabric as possible from the blind to avoid creasing the fabric)
2. With the tool supplied...or just using the black end bracket ... wind the spring up with your right hand by turning it the set number of rotations based on the instructions below...until it wont turn any more.. (keep a tight hold of this tool once the spring is wound up)
3. Now the critical bit !
While keeping a tight grip of the tool in your right hand, rock the roller back and forth as fast as possible to lock the tension in place. The motion required is similar to how you would "rev" a motorbike. It can sometimes take a full 1-2 minutes of being rocked back and forth for the tension to lock in place so some perseverance may be required. You will know it has worked when the tool in your right hand is no longer trying to unwind itself.
Refit the blind into the black end pieces and replace into the window frame.