If your handlebar is not 26.0 mm (probably 25.4 mm, 22.0 mm is mostly found on BMX bikes), you can use an aluminum can as a shim underneath the provided plastic shim to make the bar thicker for the clamp+shim.

How to measure the bar diameter:
1.) Use a caliper.
2.) Wrap a length of string (the non-stretchy kind) around the bar. Measure the length in mm and divide by 3.14159. However, this method is prone to error, and the difference between 25.4 and 26.0 is small.