Screwing reflectors to pedals - what washers do I need, and where?

Those spring washers are normally used under the tighening nut. They are used to keep the nut in place.

The normal washer would go on the other side but I would consider them optional.

Danger danger!

I have used those reflectors and they are a piece of crap. The failure mode is the plastic breaking. Your shoes will damage them in no time. I'd expect 2000 km use out of them. Then you'll see the plastic holder is damaged and the plastic reflectors have been shattered and lost. The only things in these that don't fail are the metal parts.

If you use PD-M324 pedals, instead purchase orange ECE-R104 reflective tape. It is readily available in long lengths because in some jurisdictions trucks have to use it to mark the perimeter of the vehicle. 5 meters long x 5 cm wide costs less than 10 dollars/euros/whatever-similar-currency-you-use. You can use one roll to reflectorise 50 pairs of pedals once, or one pedal pair 50 times. Thus, one pedal pair costs 0.2 currency units to do.

The reflective tape lasts longer than the plastic reflectors, is probably about as good in reflective power, and it's much cheaper to install.

My current solutions is using PD-T8000 pedals that have the plastic reflectors factory-installed and mounted in a durable metal enclosure. They last a long time too. Besides, the benefit of PD-T8000 is that they have a "heavy side" and a "light side" so every time you stop they are in a well-determined position, so you can time your feet in such a manner that you can repeatedly hit the platform side or the cleat side. Also PD-T8000 are much lighter weight than PD-M324.

But if you want to use PD-M324 despite their shortcomings and don't wish to upgrade to PD-T8000, do use the orange ECE-R104 reflective tape.

The "normal" stacking order for those would be

Reflector       Pedal body        Flat washer      Spring washer      Nut

The flat washer spreads the load, and the spring washer is supposed to bite into the nut and the flat washer to prevent the nut from turning under vibration.

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Your other option is to do away with both washers, put a dab of threadlocker on the thread that will be under the nut, and then tighten the nut down firmly. Once the threadlocker has cured/set use a dremel or small hacksaw to cut off the extra thread for weight-weenying :)

You can also use a dab of superglue to stop the yellow plastic from sliding out of the holder over time.