Trim an oblong patch of smooth "female" velcro that will fit under the laces on the tongue of your shoe, closest to the toe area. This patch should be larger, so you can experiment with the best position for the sensor once everything is in place. You want the sensor to be as level and untilted as possible.
You will need to sew or otherwise attach the velcro to the tongue, as the adhesive on the back of the velcro probably won't be completely secure on the fabic of the tongue. You will need a needle and thread and just make 4 stitches at the corners of the patch of velcro to secure it.