Step 1. Start your engine and warm it up for about 30 seconds by revving the engine from idle to mid-throttle while holding the wheels off the ground.
Step 2. Allow the engine to idle for 10 seconds. If the low speed needle tuning is so far off that it won't idle, simply bump the idle position higher until the engine will reliably run at a higher idle.
Step 3. With the engine at idle, pinch and hold the fuel line near the carburetor, cutting off the fuel flow and carefully listen to the engine rpm.
If the low speed needle is set correctly, the engine should increase rpm only slightly and then die.
If the engine increases several hundred rpm before dying, then the low speed needle is too rich and needs to be turned in.
If the rpm doesn't increase at all and the engine simply dies, the low speed needle is set too lean and should be richened or turned out.
After doing this a few times you'll get the hang of it, and tuning the low speed needle will become easy. The only way to get that last 5% performance on the low speed tune is through careful track testing. By evaluating the engine as it accelerates from a dead stop or out of slow corners during actual race conditions is the only way to get those last few percent of power from your engine. Variables like the clutch setting, gearing, traction, atmospheric conditions and even the car's weight will have an effect on this last 5% of tuning performance, and the optimum setting can only be reached by careful on-track tuning.