Lay down a folded tea towel over a chopping board, then place the spaghetti squash on top. The tea towel will keep the squash in place while you cut it.
Place your non-dominant hand on the squash towards the stem to keep it steady. Take a small sharp knife in the other hand, starting from the top and middle, and make minor dash-like cuts along the spaghetti squash lengthwise, away from the stem and towards the end.
Turn the squash around and make similar cuts towards the opposite end from the middle. You should have a dashed line running from stem to end.
Flip the squash over and repeat with the opposite side. You should have two dash lines running along the length of the squash that lead to both ends of the squash.
Hold the squash closer to the stem with one hand and use a large sharp knife to cut into the squash from the middle. It should be easier to cut from the center down to the end in one go.
Turn the squash around, starting from the middle again, and cut down away from your hand towards the stem. Don’t try to cut through the stem.
Repeat with the opposite side of the squash along the dashed line.