Photograph reproduced under Creative Commons credit: scazza (Flickr)
My cat Binnie Do was injured once. The fact is that you might not be sure that your cat is injured or in pain. A sign that she is in pain is if she runs away and hides in a dark spot out of the way. She wants to be left alone. The usual places are under furniture if indoors and under a bush, for example, if outside.
Provided you're cat gives clear indications that she is alright at a fundamentally level then she is best left alone. For example, you may know why your cat is in pain (post operation). Make sure there is water and a litter tray near by and that she has peace and quiet for up to 24 hours.
If your cat stays put for longer you would need to take action and check her over. That would mean some upset, but needs must.
What should you do if your cat shows signs of injury? He or she will want, as mentioned to hide, and she probably be frightened. The natural reaction for your cat under these circumstances is to be aggressive even towards people she knows ( and are trying to help). Take appropriate care to protect yourself (gloves and long sleeved top).
To both protect yourself and your cat from further injury (and to better manage the situation) you will need to secure your cat. No-one I know has a suitable basket for this purpose so the best thing is to wrap your cat in a towel to the extent that movement is prevented (see picture - although this is a cat that is resting after treatment as I understand it).
You can then safely transport her to the vet as quickly as possible. If she is bleeding you will need to limit the flow of blood.
This can be done by by applying pressure to the area. If it is suitable, a pressure bandage can be used (e.g. for a leg). Otherwise applying pressure by hand is the next best thing.
Injuries such as suspected broken bones (feet and legs), burns and eyes (through a fight or if there is something in it) are best dealt with by a vet as soon as possible.
Transportation may be necessary as described above if she is in pain.