A couple more things that may help you out:
Buy some plumbers grease to lube up the stem/washer/o-rings and threads before installing them back into the mixer body
Before inserting the stem, make sure you turn the shaft to the full open position to back the washer away from the seating area. Then screw the shaft into the mixer body and tighten it in place with a wrench. Quite often people forget to unscrew the shaft when installing faucet washers. In that case, the new washer is forced into exactly the same position as the defective washer and it instantly distorts the new washer, which results in a leak.
Life is about using the whole box of crayons!