1. Copy the apk and paste it into /system/app -- you'll need to press the button on the top right that says "Mount R/W".
2. Long-press on the newly pasted apk file and select Permissions.
3. Change the permissions to User: Read - Write, Group: Read, Others: Read
4. When you click Okay, you should see "rw-r--r--" in the fine print under the apk name.
5. Click the Mount R/O button, then reboot.