The RPM Cadence Sensor is designed to mount on the inside of the non-drive side crank arm. Placing it on the outside of your crank arm can cause interference by scraping against your shoe. Likewise, installing the RPM on your drive side crank arm may rub and cause interference with your drivetrain.
When placing the RPM on your crank arm, be sure it does not make contact with the frame as your crank rotates. Once a suitable position on the inside of the non-drive side crank arm is found, use either the 3M double-sided adhesive or the silicone mount and zip ties to affix the RPM to your crank. The RPM should be flat against the crank and oriented either directly parallel or perpendicular to the crank arm.
The last option is the shoe mount - this should clip directly to a strap or between your shoestrings, in alignment with the length of the shoe as shown below:
Note: for values upwards of 150 RPM, the RPM Cadence Sensor works best when mounted as close to the bottom bracket (center axle of the crank) as possible.