Pressure ulcer (Bed sore). How can I minimize the problem?

I would want to have a fingertip pulse oximeter, because I suspect her circulatory system is failing as well. You could also check the BP (blood pressure) because low BP cannot enough circulation to tissues and they die off.
Be alert for any bad odor or blackish tissue, indicating gangrene.
If pain meds are prescribed, don't hold back.
Reposition her every 2 hours when she is awake, to prevent further damage.
Fluffy sheepskin does a great job to relieve pressure, so does an adjustable bed with a bariatric mattress

Actually, don't do donuts or count on sheepskin or creams. Pressure sore prevention requires relief of pressure that allows microcirculation and decent nutrition. Pressure sore healing requires more stringent relief of pressure, e.g. if you have one over the ischial spines, don't sit until it heals. Turning q 2 hours will work for prevention for most people, and an actual pressure reducing mattress overlay such as GeoMatt or alternating air may let you get by with less at night. Donuts do not reduce pressure where they are placed and cause some edema in the middle. Soft surfaces are better than hard, but seat cushions that really reduce pressure cannot bottom out or concentrate pressure over a small area. Three inch T-foam is a start and the best one is usually a Roho, but movement such as tilting back or weight shifting is important. Can you describe the actual wound better - exact location, size of open area, color?

Thank you for the quick reply.

I am in the process of replacing the hard mattress with a softer one.

The color of the wounded area is reddish and is turning into blackish color after we applied the cream for a few days. The size is about 1 inch in diameter. The one on the shoulder just started today.

We suspect it is blood circulation rather than sleeping for too long since she only sleeps for 10 hours a day.

oh my. Well, once you have an open sore, your best chance of getting any healing is turning her so there is no pressure at all on the area. Black eschar mat have to be debrided and that could need a visit to a wound care specialist if no one who comes to the home could do it. Is she still able to get good nourishment with protein foods, or is her condition worse than that? Sorry this is happening...