I wanted to mount the heating element as close to the bottom of the keg as possible so I could heat up smaller volumes of sparge water if needed. Of course the bottom of the keg is curved, so go too low and the o-ring will not make a tight seal. I marked where I felt comfortable and drilled a 1 1/4" hole with the step bit. I threaded the element through and tightened the 1" stainless lock washer down on the inside. After tightening I added a ring of silicone around the edge of the element as an extra precaution. This shouldn't come in contact with the wort, so again I wasn't overly concerned about food grade.
Secure each of the hot wires to the heating element. You want the ground to be secured to both the handy box and to the brew kettle itself. I mounted the ground wire directly to a machine screw I put through the bottom rim of the kettle and ran a short strand of 10G wire to connect it to the mounting point inside of the box. Check for continuity between the grounding prong on the plug and the kettle.
Drill holes for the temperature probe and/or sight gauge and mount those as well. If doing both a sight gauge and probe, I recommend using the parts listed earlier to save drilling another hole. Less possible leak points are never a bad thing. The probe needs to be calibrated and it's way easier to do before you mount it to the keg (see next step). You will also want to calibrate the sight gauge by adding water a half gallon at a time and using a marker to mark the volumes on the gauge.