Because we don't know the population standard deviation, we will use the t-distribution.
To get a numerical answer using a TI-83 or TI-84 calculator, use Stat->Tests->T-Interval. Chose Stats (as opposed to Data, since we already have the sample mean and sample standard deviation statistics, rather than just raw data).
Enter the mean as x-bar,
sample standard deviation as Sx,
the sample size as n,
and the confidence level as C-Level.
The result should be that the 90% confidence interval is (25.37 ounces, 26.63 ounces)
Let me know if you would also like to know how to do this "by hand".