Grab a baggie full! Grab one for me too!
I'd rinse it really well because you'll need a couple of fish in your duckweed container to provide fertilizer for the duckweed and I'd try to reduce the chance that some parasite from the pond will affect the fish. I purchased a couple of cheap cold water fish (i.e. goldfish) so I wouldn't have to heat the aquarium.
Get your container set up before collecting your duckweed as you'll want to allow the chlorine to dissipate if your water is chlorinated and allow the temperature of the water to stabilize.
After your container has been set up for a day or two add your fish. I like gold fish because they'll eat the duckweed. I figure add an insect here & there and hopefully the need for supplemental food for the fish will be minimal.
I waited another day before adding my duckweed. There still isn't a lot of nourishment in my water for the duckweed yet so it's multiplying slowly. I'll likely add another goldfish or two to try to dirty up my tank.
Also it will take time for the bacteria to get established to convert the raw fish waste into a usable form for the duckweed. Or you can buy a starter culture to jump start the process.
I think bacteria to convert the ammonia into nitrates to feed the duckweed is what you want. It's said that goldfish produce 3 times the ammonia as other tropical fish of the same size. So they're a good source of raw material for feeding your duckweed. And the fish will eat some duckweed so a good circle of life.
Good luck!