I started mine when she was about 12" tall, and went gently, and gradually increased the tension.
You don't need anything to 'hold the string, you tie a loop at one end, large enough to slip over all your leaves on the branch, slide the loop around the branch (make sure it's not a slipknot so it doesn't tighten up and constrict around the branch. Here's a link to a fairly easy end loop, easier than the one I use, but this will work for you.)
Then take the other end of the string, and tie it through whatever hole you want.
Here is a pic showing a pretty good shot of the holes I drilled in the 1 gallon bucket I put her in first.
Sorry, I forgot your water question.
That's a call you will have to make.
When my soil is like you describe, I always go ahead and water, because I don't like the soil to get completely dried out.
When she was in the smaller bucket, I was watering about every third or fourth day, so ya, 96hrs is not out of the question.
Here's one that shows good in the 5 gal. that she is in now.
See how I have the row of holes around the top and the strings tied there?
Sorry, forgot the link after all that.
http://www.netknots.com/rope_knots/bowline/