Start by jacking up the car using the central jack point. There are two small parallel metal plates directly behind the plastic undershield.
Don't forget to set your parking brake and chock your rear wheels too.
You may wish to use a block of wood to help protect the metal plates. I originally used a 2x4 but after dropping the suspension 2", the 2x4 was too thick b/c I have a large jack so I switched to a 1" thick piece of wood.
After jacking up the car, support it with jack stands on the side.
Then open the hood and loosen the top strut nuts, but do not remove them yet.
Remove your wheels to access the suspension.
Here is the front left suspension from the front.
Note the ABS Sensor wire. Remove the 12mm bolt to release the wire.
Here is the suspension from the back.
Note the brake line. Remove the 12mm bolt to release the line.
Mark the top strut bolt with a marker. This bolt is the camber bolt so marking this will let you keep the same camber when you put in the new strut (see next step).
The front is easier than the rear because you have more space to use a socket wrench on the lower bolts, or even better an impact wrench.
Here is a close-up of the camber bolt on the front suspension. As you can see, the bolt is not a perfect circle: it is basically a cam so as you rotate it, it shifts the wheel hub, altering the camber of the wheels.
Remove the top nuts and you will be able to remove the strut assembly. Here is the stock strut compared to the WRX strut, which is almost 2" shorter.
Reverse the process to reinstall the new strut (replace top nuts, then bottom bolts making sure to align the camber mark you made, replace brake line and replace the abs sensor).
