When you install an
aftermarket one, they come with a guide about the placement and sizing. If you place it in front of the air conditioning condensor, radiator etc, you can go with a smaller one. If you place it behind all that, where it gets less and warmer air, you need a larger one. They come with some length of the proper rated hose and some hose barb fittings usually. The mounting systems sometimes come with tabs that can be inserted through the fins on the radiator. Or else you need to screw it to something.
What you normally do is from your outlet line on your factory transmission cooler, you cut the hose right after it comes out of the radiator. Then you install a hose barb to hose barb connection of the correct size with hose clamps. The other end of the hose goes to the inlet on your transmission cooler. The outlet from that transmission cooler then goes to the transmission side of the line you cut in half. That completes the loop.
Alternatively, you can just completely bypass your factory transmission cooler and go directly into the
aftermarket one. The fittings that go into the transmission cooler are generally not something you could get an adapter for however so you would still need to cut them off. Thats kind of annoying.
Overall, its a messy process and I don't like hose clamps at all for anything. Its not my favorite mod because of all that.
To do it right, you also need a thermowax or thermostat for the transmission cooler. The transmission needs to warm up and having cold ATF circulating through the cooler all the time may do more harm than good. The factory cooler kind of achieves this one way or another. Typically the coolant warms the ATF because the coolant warms up more quickly.