How do you nul the resistance, of the test leads, of a Simpson 360-2 run 2 digital multimeter?

If you get any significant resistance, try different leads. They should show zero. At most a small fraction of an ohm. Try also fresh batteries...

I don't know the answer but there are manuals available for download and youtube videos illustrating the calibration procedure.

I suggest you ask google.

Test the coil electrically
A digital multimeter is required for testing the resistance levels of the ignition coil. The terminals of the coil are going to be marked "negative" and "positive," and these are the two points where the resistance of primary winding can be measured. Step 1: Prepare the multimeter
To check a 12 volt coil, set the multimeter to at least a 200 Ohms setting. Attach both leads of the meter to the coil-terminals with black to the negative terminal and red to the positive one.
Step 2: Test the resistance
The normal, acceptable range for a standard 12-volt car is 1.5 to 1.7 Ohms. See if the reading is within this range.
Step 3: Test the secondary coil resistance
Set the meter to 20K Ohm setting, and attach the leads to the ignition coil's center terminal. The reading for secondary-coil resistance should be 11, because if it is lower

A digital multimeter is required for testing the resistance levels of the ignition coil. The terminals of the coil are going to be marked "negative" and "positive," and these are the two points where the resistance of primary winding can be measured. Step 1: Prepare the multimeter
To check a 12 volt coil, set the multimeter to at least a 200 Ohms setting. Attach both leads of the meter to the coil-terminals with black to the negative terminal and red to the positive one.
Step 2: Test the resistance
The normal, acceptable range for a standard 12-volt car is 1.5 to 1.7 Ohms. See if the reading is within this range.
Step 3: Test the secondary coil resistance
Set the meter to 20K Ohm setting, and attach the leads to the ignition coil's center terminal. The reading for secondary-coil resistance should be 11, because if it is lower, that's the reason why sparks are not appearing