1. “Insole Sizing: The insoles are designed to be trimmed before use (trim-to-fit); the insole is manufactured to be used in a range of shoe sizes.

2. Insole Placement: If you purchase a full-length insole, you will most likely need to remove the existing insole from your shoe before placing your new one. Almost all full-length insoles are meant to completely replace the insole you are currently wearing. 

3. Your Foot Arch Type: The arch of one's foot usually conforms to one of three different arch types: 1) neutral or medium arches, 2) low arches, flat feet, or fallen arches, and 3) high arches. Similarly, every insole is designed to work with one or more of these foot arch types. When browsing insoles, you should first identify what your foot arch type is and only browse insoles that are designed to work with that foot arch type.

Wearing an insole that is not designed for your arch type will likely be painful! 

1. Insole Footbed Type: In general, insoles and orthotic arch supports have one of four different footbed constructions: 1) rigid orthotic arch support, 2) semi-rigid orthotic arch support, 3) cushioned arch support, and 4) no arch support/flat cushion. The type of footbed that you need will be heavily dependent on why you are looking for an insole. 

2. Material: The four most common materials from which insoles are made are foam, gel, cork, and leather. Each has their advantages, and the material you choose is largely based on preference. In general, however, foam works best for cushioning, support, and pressure relief; gel works well for shock absorption; cork works well for support and slight cushion; and leather works well for cushion and "feel" (especially when worn with thin socks).”

Ref:http://www.insolesandbeyond.com/collections/insoles-for-overpronation-orthotics-for-pronation-what-is-overpronation-of-the-foot-best-orthotics-for-overpronation-1

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