The cable guide is designed to provide a 'clipping' point for the power and thermister cables that are attached to the hotbed.
Without the clip during bed movement the cable tends to bend at the solder points which over time will weaken and break.
The cable guide isolates the solder points from any movement.
Measure the diameter of the cable/cables that connect to your hot bed.
Open the scad file and adjust the cableRadius variable to be half that of the cable diameter.
Generate the STL file and hit print.
The Cable Guide prints with a small support surface between the two walls of the guide (it's the cross bar in the H shape that you will see at the start of the print). After printing you will need to cut the support surface out.
If you want you can drill a hole through you print bed and use the M3 hole in the cable guide to bolt it to your bed. I didn't bother as the pressure fit of the guide was sufficient to hold it in place.
I've attached STL files to suit a variety of cable diameters. Of of the STL's assume a 6 mm hotbed.