Just wash off the chemical soon as possible. It will turn red. And then it will go away. If it bubbles up, then wrap something around it to absorb the liquid that explodes from the rash.
This is a bit of an oversimplification. I am probably on the more sensitive end of the spectrum when it comes to poison ivy/oak but I have gotten rashes that are legitimately debilitating (i.e. typing was very painful). This was also after washing with dish soap post-exposure. I've since switched to Tecnu and it seems to work better but, unfortunately, is ridiculously expensive.