Concept- The arch piece is designed to support the foot by cradling the forward foot arch.
Start with the lowest arch piece (L1) and progressively move as far forward as is comfortable, overtime continues to move the arch piece forward. Once all the forward positions are exhausted it is now time to select a higher arch piece, starting midway again then progressively moving forward until the arch piece is flush forward supporting the metatarsal arch.
The highest arch you use will be governed by foot structure and anatomy eg; If you are flatfooted you might stay on L1 or progress to L2. If you have a naturally high arch you may start on L2 and progress up to L3, L4 or L5