There are several ways to do this....Select which is most comfortable for you and your cat.

Holistic Method...Add 1/4 to 1/2 teaspoon per serving depending upon the amount of food fed to your cat, of pure pumpkin, canned or cooked.  Be certain it does not contain the "spices" in it for pumpkin pie.  Do this a few times a week and you will not have any problems with hairballs.  I've done this for   a few months now and it does work beautifully.

Hairball Management Canned Cat Food...As you are aware, I do not advocate canned food, but this may work for those of you who do feed canned cat food.  My older cat was having severe hairball problems, before I began feeding her the pumpkin and I tried the hairball management canned food.  She ate it for about 2 weeks, I saw no improvement in the hairball situation, then she refused to eat it.  Hence, my search begin once again.

Hairball Gel....This contains no petroleum, no mineral oil,  which interferes with the cat's ability to absorb nutrients from its food.  It has canola oil, vitamins, antioxidants, and taurine while assisting in the removal of hairballs.  This works great...I keep it on hand for any "problem" hairballs and give it to my cats once a week.  It is gentle enough to be given daily....To order see information below....

Homeopathic Method....For those of you who practice homeopathy, I found the following helpful.    The primary remedy for expelling hair is Nux Vomica.  Use it twice a day for a few days or by the observation method, to remove hair by vomiting or the stool.  If vomiting causes pain to the cat, try Phosphorus.

I hope one of these methods will prove helpful in your effor to assist the cat in its hairball elimination process.