Hold the wrist strap in your hand and make a larger knot by inserting flat end of the strap into the loop at the other end of the strap.
The other hand
Repeat the same with the other hand and make sure that the logo of the strap appears on the back of your palm right between the fingers and wrist area.
Position of the Straps
After you have the straps on both hands then ensure that the position of the wrist straps is exactly the same on both your hands so as to avoid any balancing problem. The loose ends of the straps should be right along the thumb and forefingers at equal lengths.
Placing you hand on barbell
Once you are sure that the both the straps are tightened firmly around both your wrists then place your hand on the barbell bar but with less force.
The loop on barbell
Using the free end of the strap and create a double loop around the bar. As you close your hand, the loops should be right under your palm such that the hand are holding the strap instead of the bar.
All set to lift weight
Repeat the above process for the other hand and before you are all set to lift the weight try rotating the bar towards you as it will tighten the loops more.