I just did the timing belts on my 04' kia optima back in late november. I'm including a link that shows the timing diagram. First things first, to make it easiest, get all the timing marks lined up BEFORE you take off the belt. This is very important. You'll have to use a large socket wrench on the crankshaft to turn it manually until the marks line up (helps to have a second set of eyes for this).
As far as how to find the marks, for the camshafts up top, you're looking for a little V shaped nub on the casing. Should be directly above each camshaft sprocket. Then look for a notch in the camshaft sprocket. Line those up, and most of the rest should fall into place.
You will still need to check other marks. On the oil pump sprocket (just to the right of the crackshaft), there will be a V shaped notch on the sprocket and a V shaped mark on the engine housing. Verify they line up. Look for the same kind of marks on the crankshaft (which should be lined up anyway if the cams are lined up properly) and don't forget the balance shaft belt (seperate belt behind the crank sprocket that goes up to the left) which will also have similar marks. As a side note, my balance shaft belt was completely stripped, so I had to manually turn it until it lined up.
Once everything is lined up, you can remove the belt by removing the belt tensioner.
SAVE THE OLD BELT!!!
When you proceed to put the new belt on, you will notice that the two cam shafts have both lost their alignment slightly. They will have both turned in by one or two sprocket teeth. Unless you have a special tool, this is where the old belt comes in handy (as was my case). You will also need a second person.
Cut the old belt so it is one long strip. Your friend needs to have two large screw drivers ready (take this next advice with a grain of salt, much consideration and good judgement on your part). Start by having the new belt ready to be put on the the sprockets (your balance shaft belt should already be on as well). You should have a cotter pin or something holding the tensioner open so that when you're ready, you can pull the pin out and be ready. Using the old belt to pull the camshaft sprockets into proper position, have your friend use a screwdriver to hold them each in place (this is somewhat difficult, so like I said, have the new belt in a ready position to be quickly put on). Once your friend is holding them, start putting the belt on. Once the belt is in position release the tensioner.
Next, CHECK all of the timing marks to ensure that they're all still lined up. I say this, because my friend and I had to do this step twice to correct for an alignment issue we had with the oil pump.
If everything lines up, then you are ready to start putting it all back together. Good luck to you, and like I said, take the screwdriver and camshaft part with a grain of salt. It's what we did and it worked, but it's definitely not the recommended way to go about it.
Here's the link to a page on the website www.2carpros.com that relates specifically to your issue and has diagrams. It's what I used and it helped tremendously!http://www.2carpros.com/questions/kia-optima-2004-kia-optima-help--3