In a bucket.
Fill it half way up with sand, stick a hose in, and pour off anything that doesnt immediately sink.
Or my method;
Puncture several hundred holes in the bag (a fork will do), put the whole bag upright in the bucket, and cut a hole big enough for the hose to go in the top.
Turn the hose on and leave in the garden for several hours. (Might be worth doing half a bucket full wit the above method after to check)