Use your 1/4 teaspoon measure and fill it half full. That will be 1/8 of a teaspoon.
I Need To Measure 1/8 Teaspoon Of Salt And Do Not Have A 1/8 Spoon To Measure, How Do I Measure This?
Well, basically try your best to estimate it and it is better to put not enough salt then too much. :)
Assuming you have a teaspoon: Measure out the teaspoon take out a quarter and half this amount.