This may be a repost, but I just got these instructions for reprograming WRX "round remotes" when I purchased a used one off of ebay. Hopefully they can be helpful.
Round Remote Transmitter Programming Instructions
1. Disarm the security system (if so equipped).
2. Open the driver's door and sit in the driver's seat.
3. Close the driver's door. 4. Place the ignition key in the ignition switch and cycle the switch from "LOCK" to "ON" ten times within 15 seconds. Be sure to stop at the "LOCK" position and leave the key in the ignition when you are done. The horn will sound once to indicate that you are in transmitter programming mode.
5. You must finish the next steps within 45 seconds of the horn honking.
1. Open the driver's door.
2. Close the driver's door.
3. Press and release any button on the transmitter. The horn will sound two times to indicate that the transmitter has been programmed.
6. After programming your remote, remove the key from the ignition switch. The horn will sound three times to indicate the system is now out of programming mode. Ensure your remotes work properly at this time
You may program up to four remotes at any one time. If you lose a remote, program the other four times to clear the code for the lost remote until you can replace it. Do not program your remotes with other Subarus in the vicinity- you may be unknowingly programming someone else's remote for your vehicle!