Step 3: Attach the mounting plate, heatsink, and LED plate together
First step is to drill and tap the center bottom of your heatsink. Most likely, this will be a "blind hole" meaning it doesn't go all the way through (since there is a fan in the way), so be extra careful with the tap.
Spread some thermal paste on the heatsink surface (the thinner the better, as long as you cover everything), place the block on top, and screw it into place tightly using the center hole.
Spread some more thermal paste on the block again and put the LED plate on top. Screw it in tightly and you're done.