How do I change the oil in a 1999 Rodeo and add transmission fluid?

So here's the thing: there are two pans under the truck, my husband drained the wrong one and we think it was the transmission fluid. It has a hard time shifting now - we only drove it to see if that was the problem. How do we fix this problem? He added the new oil - so we are now overflowing on oil as well. Where do we add the transmission fluid? Is there anything else I should know? Please don't let me lose my favorite ride!!! Help me!