Step 1) Now that you have removed the secondary windings, shunts and filament windings and its as tidy as possible, you can take your new secondary wire (whatever gauge your using) and wind as many turns as possible on your transformer as you can fit, the more turns the more volts. and the thicker the wire the more current you will get. to wind the new secondary, just wrap your new wire round the core of the transformer where the old secondary was.
Step 2) Now you can attach whatever you want to your new secondary's two output leads, whether it be some big heavy duty alligator clamps or connecting it up for use as a spot welder or stick welder
Step 3) Get your 3 pin grounded wall plug and connect the ground to the base of your transformer, and the hot wire and the neutral wire to your transformers primary inputs.
Step 4) Plug it in, turn it on and you have yourself a power supply that theoretically should be able to supply up too a kilowatt of raw output power!
And of course a video. 2-3 turns approx 3v AC and around 500 amps of steel melting power...
Heres a snapshot of one rewound MOT i put a bit more effort into. I wound it into a power supply that has 5 voltage taps which once rectified the voltage on each is 0v (GND), 12v, 24v, 30v and 50v DC! all of the voltages besides the 50v can put out 30 amps+ and the 50v limited to around 20 amps.