Lay the replacement ear cushion on top of the earphone.

The plastic bottom edge of the ear cushion should align with the circumference of the earphone.

Using an iFixit opening tool (or your finger or nail), push down on the plastic edge of the ear cushion to fit it under the nine plastic fasteners on the inside of the earphone.

After the ear cushion is in place, the nine fasteners should be visible if you pull the cushion to the center of the earphone.