Purchase a bag of duckweed at a pet store or garden supply store that carries aquatic plants.
Add fish to your pond before you add duckweed to it. If you want other aquatic plants, such as pond lilies, also add those before you introduce duckweed.
Fill a bucket with your pond water. Empty your purchased duckweed into the bucket, to prepare the duckweed for its new environment. Leave the duckweed in the bucket for up to one week to allow it to adjust and begin to multiply.
Scoop the duckweed from the bucket with a fishnet, and transfer it to your pond.
Grow more duckweed in your bucket if you want to ensure that you will have more, in case your fish eat the entire first batch.