hmmm. i'm having trouble reposting what dave attached earlier. The slow sand water filter document. (it's in 'building rain barrels').
After building a couple filters last saturday, our group (including dan borba our local rainwater guru) would make some slight modifications from the document we followed.
1) Where the pipe comes out of the bucket it leaks around the hole. The remedy is a $3 bulkead fitting.
2) The water moves way too fast out of the top bucket to the bottom filter, so spilling over the lid instead of going down. Instead of the six holes recommended in the guide, we did a bucket with just one hole and we all agreed that works much better.