The pedals have a letter indicating which side of the bike they install onto. Pedals are marked “L” and “R.” The left side goes on the right Crank. The right side goes on the right Crank.
Pedals are not designed equally. Please note the left pedal is reverse threaded. You’ll have to turn it counter-clockwise to tighten it. This is so the pedal doesn’t unscrew itself while you’re pedaling.
If you had threaded the pedals and the washers snugly onto the drive-side crank arm, please remember to remove the washers.