Start with quality freshly cut logs for your cut siding. They should be of similar diameter, for similar siding widths, but varying lengths are fine.
Secure the first log and set the sawmill to the siding profile and depth you prefer. The rounded log look is very popular; the thicker the siding width is, the more realistic the look. Commercial log siding thicknesses start at about 1.5 inches and can be greater than 5 inches.
Cut the first rounded side from the entire length of the log, creating the first section of siding. Reposition the log to cut another length of siding. Depending on the log size and the siding profile, you will be able to cut two to four lengths of siding per log.
Continue cutting siding from the rest of the logs following the same process. Set aside the unused remainder of each log for later milling into flat lumber.
Finish milling the log siding sections on a table saw to create tongue-and-groove fittings. Set the height of the saw blade to match the depth of the groove you want to make. Set the table saw’s fence to cut the tongue’s upper face. Run all of the log siding sections through the saw, with their undersides pressed against the fence.
Adjust the table saw fence, moving it closer to the saw blade, so it will cut the tongue’s lower face. Run each log board -- in the same direction, underside down -- over the blade one more time. Using a narrow chisel, clean out the groove in each board.