It is mainly drop in. Just be sure to get a pail under there or you will have a huge toxic mess on your hands from the antifreeze. Also be sure you fully burp the system once you have it up. If you are going from a one cap setup to two caps you will need to T in the lines. If it is Mishimoto or analogous you will have to trim the hoses - just go slow from both ends until it clears everything. Also, the bottom hose/clamp may be easier to attach to the radiator and clamp on before you install the shroud.
How To: Install aftermarket radiator (Koyo racing rad), and flush the coolant - IWSTI.com: Subaru WRX STI Forums
Oh, and don't cut the shroud like indicated unless you need to.