ok, firstly why do you want to run an external wastegate?
secondly it's not so simple lol.
you'll need to cut a hole on the base of the erst manifold and weld a pipe into it before the turbo where all the pipes meet. then run this pipe off and fit a flange for the external wastegate, then a pipe out of this to feed back into the exhaust or just out of the engine bay to wherever.

a wastegate bleeds off exhaust gas this inturn reduces the flow of gas to the exhaust turbine in the turbo, this in turn reduces the boost. if the wastegate is closed it'll just keep spooling and producing big boost, but as a normal wastegate there is a feed off the boost pipes which opens the wastegate to limit the boost.
you vary how these work with varying spring rates in the unit and a boost contoller if you wish.

they are better than internal ones as they help the turbo flow better and react better to boost spikes etc.

p.s. welding steel to cast iron isn't easy and commonly fails due to the intense heat heeded and the slow cooling to make it stay intact and not crack later.