How do u do an open water swim

I would suggest getting with a local tri or swim club and see if they have any scheduled.  If they don't try to find a friend that has a canoe or kayak that can go out and support you.

the august 'triathlete' has an article on some prep work you can do in the pool for open water swimming.
your local triathlon or masters group would probably be the best place to find some others to swim with.
there might even be an open water club....

It's not easy!!! It is very confusing, and I got stung by a jellyfish mid-race. I would recommend practicing in the open water because the current and temperature really threw me off, and also practice looking for the buoy. Don't forget that if you get overwhelmed you can just switch to breaststroke or side stroke until you calm down. I had to do that a couple of times.

You need to be able to swim comfortably for at least 500-1000 yds before you go jump into the open water.
1st - get into the pool and practice

Then, once you are comfortable in the pool and with the basic swimming technique, check in your area for open water swim clinics. These are the best way to get your "feet wet" Usually, there's a coach and you go out and practice with others. It's a really good intro.

once i got my distance up in a pool, some friends and I did a mock tri with the swimming part the distance of an up and comming tri we were all competing in. This was good except the conditions in our mock tri were too good which is exceptional ultimately the tri I was in had high winds with nasty chop and waves, I did is sans wetsuit which is something I wish I had.
so maybe
-get a wetsuit
-choose conditions that are challenging for openwater practice
-don't practice alone!