Giving cat medicine can be a pain in the ***. Cats can be very difficult when you want to get a pill down or give her some liquid medicine. It's a huge put off and you are liable to not give all the medicine prescribed.
Of course you must do as your vet ordered :) Vets will prescribe pills or liquid medicine.
The classic way to deliver these is to firstly hold your cat's head firmly. I think that you have to demonstrate to your cat that you are the boss and that must come across in a firm hold but obviously not too firm. It will help is you have someone to assist to keep her still and further send the message that you both mean business and that it is better to submit. That should make the process easier. But always act with caution and sensitivity to the cat's feelings.
With the head held firmly, you pry (gently but firmly force open the mouth) with a finger of the other hand. Speed is important too. The faster the easier. If you take too long the moment is passed and your darling cat will win that one.
If you fail, I would stop and come back for a second attempt a bit later. Once the mouth is open, drop the pill at the back of the mouth. The pill should be pushed far enough back on the tongue for the automatic triggering mechanism of the cat to take effect. She may and probably will try and spit it out.
Experience will improve your technique. Apparently, a little tap on your cat's nostril (be careful here) can help to trigger the swallowing process.
If this fails you've got the pill dispensing devices. You can get pill crushers and pill dispensers both sides of the Atlantic. They are cheap (in the US below $3) and you can buy online. Of course they are probably twice that price in the UK :)
The crusher of course allows you to put the crushed pill (now powder) into some food (something that she can't resist). I find this method one of the best, in fact. Of course you can crush the pill yourself.
The dispenser (pill gun) works by loading the pill on the tip of the device, opening your cat's mouth and pressing the tip of the device gently to the back of your cat's tongue and releasing the pill with the trigger on the device. Voila!
If you have a syringe (perhaps given to you by your vet to dispense liquid medicine or on request - without the needle) you can give your cat a quick drink of water with it to wash the nasty taste down.
This is dispensed with a syringe. These are marked up on the side in cubic centimeters to make it easy to gauge the quantity of medicine dispensed.
Hold head and open mouth as before and place syringe at back of mouth and plunge gently and in a regular manner to allow your cat to swallow the liquid.
Photos: top dave77459 and bottom: Jan Steen (painting) and mharrsch the photo both creative commons Flickr