Making your cat physically comfortable is an important part of easing her passing. Medications such as painkillers can be helpful. Talk to your cat's veterinarian to determine what medications may be useful.
There is more to making your cat comfortable than seeing to her physical needs. Providing loving attention and talking to your cat in a soothing voice can be helpful. It's debatable whether or not cats understand words, but the emotion behind what you're saying can convey plenty of meaning.
Cats often hide signs of illness or discomfort, so it is important to pay attention to changes in behavior to determine how best to help your cat. If she is having trouble moving, then ensure that she has easy access to food, water and litter box. As her health declines, you may need to soften hard food with water or gradually switch to a soft food diet. You may also need to ensure hydration with an eyedropper as her mobility declines. Incontinence may also become an issue. Wherever your cat ends up, make sure it's a location that is easy to clean and that you can switch out soiled blankets with clean ones.
Ensuring your cat has a quiet, soft, and warm place to rest is another key piece of making her comfortable. If she is well enough to find a place on her own, then just add extra blankets and let her be. Otherwise, set up a cat bed or nest of blankets for her and find someplace where there isn't too much activity. In cooler weather, adding a heating pad or hot water bottle to her bed is an excellent idea. If she's used to being outdoors, then find a comfortable spot under some foliage for her to nap during the day.