Tung oil is one of the easiest wood finishing techniques. First make sure the wood is sanded down with 400 grit sandpaper. Some people like to go a step further and sand down to 600 grit but, I find that 400 gives very good results. These are shipped sanded to 150 grit. You'll need to do a 220 then 320 and finally 400 grit final sanding before applying a finish.
After all the sanding is completed you need to make sure that there is not any dust remaining on the wood. Wipe it clean with a soft clean rag. Now, you are ready to apply the first coat of tung oil.
I like to use tung oil finish made by Formby's or Minwax (available at most home improvement centers). Simply use a dry clean soft rag such as an old T-shirt and fold it over several times. You may want to use latex rubber gloves so you don't get the oil on your skin.
Simply pour a small amount of the oil on the rag and then wipe it on to the wood. Once you get one of the surfaces done you'll wipe the length of the wood going with the grain to make sure that it is evenly coated. Do this on all 4 surfaces. Now you'll want to let it dry overnight. Some times in dry climate you can get away with 10 - 12 hours drying time.
Once the first coat is dried you'll need to rub the wood down with some #0000 steel wool (available at most home improvement centers) to make a nice smooth surface. Wipe the cab down again with a clean soft rag and apply another coat of tung oil in the same manner as the first coat. Once the second coat is dry you can rub it down again with the steel wool so that you don't feel any roughness in the finish. Keep repeating these steps until you get the desired look that you want. Some people like a satin look that may only require 2 - 3 coats. Some people like a thicker looking sheen and this may require 8 - 10 coats or more.
At any time in the future if the cabinet begins to look a bit dull or there are some scuff marks you can rub it down with steel wool again and reapply tung oil to bring it back to looking new again.
Thanks and happy finishing.