The leads of the heater resistor should not protrude far beyond the edge of the heat spreader. The heat spreader provides some protection for the leads. From this location, you can eye-ball the location of the screw hole, or you can mark the center of the hole with a pencil.
Mark the hole location with a spring-loaded center punch.
Press down on the punch until the spring snaps, leaving a round dent in the surface of the heat spreader.
The dent from the centerpunch should be obvious. It only needs to be large enough that the drill bit (next step) doesn't wander when you start drilling the pilot hole in the spreader.
Secure the heat spreader in the vise before drilling
After drilling the pilot hole, there is likely to be a burr on the side of the spreader where the drill bit exited.
Change the position of the spreader in the vise, and use a file to remove the burr.
Make sure the surface of the spreader protrudes above the vise jaws. You don't want to dull the file or scratch the vise by filing the vise instead of the burr.
Here is a photo of a heat spreader with the burr removed. It's OK to accidentally round the corners of the spreader, which is made of (relatively) soft Aluminum. The important result is that there are not sharp protrusions that may tear or puncture the heat shrink applied at the end of the assembly process.
Secure the resistor to the heat spreader with the small screw. The screw should be snug. Be careful not to over-tighten the screw as you may strip the soft Aluminum.
The screw will protrude from the back side of the spreader. Flip the resistor/spreader assembly in the vise and use a file to remove the point of the screw. Again, the goal is to remove any sharp features that may damage the heat shrink applied at the end of the assembly process.
Move the heater assembly to a solder station and solder the lead wires to the resistor. Be careful not to overheat the resistor. In the photo, a heat sink is attached to the resistor leads near the base of the resistor.
Add heat shrink to the exposed resistor leads.
Before proceeding with the assembly, use a multimeter to verify continuity of the heater assembly. You want to make sure that the leads have been attached securely and that the resistor is still functioning.
STOP HERE: Ask the instructor about adding the sealant to the wires before the heating the heat shrink.
Locate the heat spreader in the middle of the large shrink tubing. The first image shows that roughly equal amounts of heat shrink tubing extend beyond each end of the heater assembly.
Consult with the instructor about adding sealant to the end of the heat shrink that has the wire leads protruding.
After adding sealant to the wire ends, work quickly (no rush, just don't dally) to use a heat gun to shrink the tubing on the end with the protruding wires. Apply the heat first to the center (at the heater assembly) and then work toward the open end. Some sealant may ooze out of the open end.
The important feature is that there are no openings for water to enter inside the heat shrink along the wires.
When the wire-end of the heat shrink tubing is finished, apply heat to the open end. While the open end is still warm, place the tubing in a vise and crush the tubing gently. You do not need to apply too much force with the vise.
Use a multimeter again to test the continuity of the heater assembly.