i believe the owner's manual has a section on fork oil changing.
evilsteve is right on with draining it first to measure. it's pretty easy if you have the right setup.
chock up the bike and remove the front wheel. there is a $2 tool to lock the axle. it is not hard to do, it takes me about 2 minutes to do now.
make sure you locked the forks straight forward (with your key).
there is a screw at the bottom end of both forks. do one at a time. put a large measuring cup under the fork. i have a 2 quart one that i use, it is a tupperware cooking type. the owner's manual says the S2R has 450 mL per tube, but you will probably get less when you drain it.
remove the screw and the oil will drain out. it is not necessary to drain it bone dry, but let it sit for at least an hour or so.
Ducati says to use Shell Advance 7.5 or Donax TA fork oil. I use Spectro. Marzocchi also has an oil available. You can probably use Wesson oil and be fine (just kidding) but we used to use ATF fluid as it is 125 gear oil.
Basically, you want to drain it the best you can, then put the screw back in and measure the amount of oil that came out.
You will put back in this same amount plus about 5% more. You don't need much. Adding oil is going to affect the last amount of fork travel. If you add too much, you get hydraulic lock so do NOT add too much. Can't hurt to keep a log for your use.
The fork oil measuring tool is useful, but it's really just a plastic graduated syringe. You can pick up a veterinary 50cc syringe for cheap ($5-
. I got one free from my vet, I just had to clean it first
The point of the syringe is to load an exact amount of oil and squirt it into the tube. You can also just re-use the oil that came out and add additional to it, as Ducati says to change it at 12k miles, but I would just use new.
A 50cc vet syringe: