1) Depress the spring plug that protrudes from the bottom of the basepad and slide off the basepad.
2) Remove the factory spring plug and replace with the new aluminum retainer.
3) Compress spring & plug into magazine body and slide on the basepad.
Installation Instructions
* Due to the nature of this item and international regulations (ITAR) we can not sell to anyone that is not in the United States. All international orders will be canceled. To place an international order, the order must be placed through an exporter. Contact an exporter HERE
** This item CAN NOT be shipped to California
*** Verify your local and state laws for any restrictions before installing these parts. It is the users responsibility to verify any legal conflict with the use of these parts.
Works great. 
Sig Sauer P28 Mag Extensions