Standoffs are used to keep the PCB off of the large metal drawer bottom. This has two effects. It results in a smaller contact area that prevents the drawer from acting like a heat sink. It also raises the PCB up so that it is closer to the thermocouples and should make the temperature more accurate.
This is one of the simplest, and arguably most beneficial, modifications. The screws are placed through the existing slots in the drawer bottom and tightened up. You don't want to over tighten and damage the drawer, but I do recommend using tools and not just hand tightening. Lock washers would be beneficial. The repeated heat cycles have a tendency to loosen the screws.
I used some #6 by 1 inch for the sole reason that I had some left over from another project.