I got this off a cat website:
Cat Urine cleaner - How to clean pet urine odors from carpet using household products
You need : Black Light
Ingredients
Baking Soda
White Vinegar
Dishwashing Detergent
3 % Hydrogen Peroxide
Directions:
1. If the carpet is newly soiled, first absorb as much of the cat urine as possible using paper towels or an old towel. Place towels over the cat urine area and tread on them so as to absorb as much wetness as possible. Repeat with dry towels until no more moisture can be absorbed.
If the cat urine has dried and you are not sure where the soiled area is you can use a black light (pictured above) to detect it. In a darkened room the black light will pick up urine and other stains. Hand held black lights can be purchased for between $15 to $25
2. Next, wet the area with a solution of 50% white vinegar and 50% water. Make sure you use enough of the solution to penetrate the fibres deep down. Allow it to dry. You can assist drying by blotting with paper towels as described above. A fan can also be used to assist drying. The acidity of the vinegar will neutralize the ammonia in the cat urine.
3. When it's dry, apply a liberal amount of baking soda over the affected area and drizzle it with a quarter of a cup of hydrogen peroxide mixed with a teaspoon of dishwashing detergent. Work it in with a scrubbing brush or your fingers (be sure to wear rubber gloves) to dissolve the baking soda and work it down into the carpet. Allow it to dry. Then vacuum.
The vinegar will neutralize the ammonia and hydrogen peroxide is a powerful oxidizer capable of killing bacteria which cause the urine smells. Baking soda is a well known deodorizer which absorbs odors. Sprinkle it on your carpet monthly then vacuum up to keep your carpet smelling fresh all the time.
Important. Never use ammonia or ammonia-based products on the carpet. One of the ingredients of urine is ammonia and your cat may well be encouraged to re-offend in the same area if it detects the smell of ammonia. Many household cleaner cleaners contain ammonia so be sure to read the label.
You may have to repeat the above process a few times for heavily soiled areas.
Caution, you should test your carpet for color fastness in an inconspicuous area for 24 hours.
Use only 3% Hydrogen Peroxide
Hope this helps.