Note on the right end it says "wind" clockwise.
hold the bar and material tightly and wind the right side end clockwise until it gets quite resistant and wants to spring back.
Whilst still keeping a tight hold on the bar and the plastic end, put the roller back in its slots.
Still keeping hold of the bar, remove the tied strings and put the material back to its natural position.
Now gently release your grip, letting the blind roll back in slowly.
Job done. Welcome to high tension yumminess !
Have fun, and hope this works for you.
Ged