Get a switch that is 'single polarity dual throw' i.e.
on
off
on
There will be three positions on the back to connect the wires to.
The midddle is where you connect the wire to the solenoid.
The top and bottom positions are used for a wire to ground and the wire connected to the ecu (the other end of the solenoid wire that you cut.)
It doesn't matter which of these two go at either end.
I wired mine
ground
solenoid
ECU
this means that when the switch is pushed foward, the boost is controlled by the ECU,
when it is pushed back, the solenoid is grounded i.e. full boost always,
and when it is in the middle, it will always be low boost.
There is more about this earlier in the thread I think.