Try watercress cultivation in Google, and you'll find numerous websites giving advice. As an alternative, you could try growing American Cress. It'll grow quite happily as an ordinary garden plant without watercress's need for water, and is an excellent substitute, tasting almost the same. Usually available among the packets of vegetable seeds at your local source.
Lucky you - your own fresh watercress ! I have no expert knowledge, but assume you would have to dig a shallow trench a few inches deep which allowed the stream to flow along it and out back into the stream further on to keep it flowing freely. You'd need to be sure that the water wasn't contaiminated, then perhaps line the trench with black poythene to stop the water draining away and fill it with fresh soil. Obviously don't dig out the barrier between stream and trench until
Whoops - got cut off mid sentence! ,,,,,until you've planted some purchased watercress which is showing roots or you've sown some watercress seed in the damp soil.
I can emphasise what heathfield says about American Land Cress. I grow it in damp shady areas of my garden. The leaves are much smaller but have a nice peppery taste and it seems to have the habit of regenerating and resowing itself quite easily.
Are you asking how to divert a stream? Can't help with that I'm afraid but I grew watercress in our water garden, until it took over! I bought a bag of salad watercress from the supermarket and saw a bit had roots on it so I thought I'd give it a go - it grew rapidly!