Toyota Prius 3rd Gen Transmission Filter

There isn't a filter. Well, there may be one internal to the transmission, but there isn't one you can readily service.

It doesn't really spell out in the attachment, but you basically add fluid, with the car level, till it starts coming back out. Both bolts are 29 ft/lb torque, require a 10 mm male hex socket. Stick with the stock Toyota ATF-WS fluid; if you purchase 4 qt's or liters you'll be fine: it should start coming back out around 3.5. You'll need a funnel with about 3' of hose*, to snake down through the engine bay. Do take the fill bolt off first, just in case you have problems. It's good to get fresh washers, especially considering it's an infrequent service.

* Checked the OD on what I'm using, it's just under 15 mm. It fits with a little clearance, but pretty close to the limit.