A retainer for assembling tissue thickness compensators to a surgical stapler can comprise a grip, a first surface for supporting a first tissue thickness compensator, a second surface for supporting a second tissue thickness compensator, and clips for aligning and attaching the retainer to the surgical stapler. The clips may align and attach the retainer to a staple cartridge of the surgical instrument. The clips may align the retainer with an anvil of the surgical instrument. An insertion tool may be used in combination with the retainer to insert the retainer into the surgical stapler and to push the tissue thickness compensators against the anvil and/or the staple cartridge of the surgical instrument.