"Is it a DIY kind of thing, or is it hard to do?"
It's not that hard, you just tap into the tranny hose that runs from your radaitor back to the tranny and create another loop so that the new cooler is in between the radaitor exit tranny hose and the tranny. It is a DIY.
Might as well drain your tranny fluid too as it might be toast from all the heat. Don't put the fluid back in until you install the tranny cooler. This way when you fill it back up, run it and then add more, you have compensated for the extra capacity.
Some people take out the loop into the radaitor section and just run the tranny cooler, this might be helpful if your tranny cooler section on the radaitor might be plugged or leaking.
Good luck.