how do you find percent yield of a rxn, and how do you find mmol of something?

percent yield = 100 * (actual yield/predicted yield)
find the number of moles, and then conver it to milimeter.

millimols = mols x 1000 OR
mols = grams/molar mass OR
millimols = milligrams/molar mass.

In titrations, too, there is
milliliters x molarity = millimols.

%yield = [(actual yield)/(theoretical yield)] x 100

find the number of moles, and then conver it to milimeter.

There is a missing link here. You can't convert mols to millimeters.