Is the dress circular? Rolled hem?

How much is it being hemmed? (Inches taking off.)

Would try to duplicate the type of hem orginally used.

Prefer using regular machine not a serger.

Try this web site.
http://www.hgtv.com/hgtv/cr_sewing_basic_tips/article/0,,HGTV_3323_1399843,00.html

Thank you for your help. That is information that needed. I am shortening a floor lenght evening gown to a knee lenght so quite a bit of fabric will be removed. Should I measure from the waist or from the original hem in order to cut the material? How do I mark where I will cut on the fabric so I don't cut a jagged line?
Thanks again for your help.

As long as the orginal Hem in tack and hangs even you can measure from the bottom.

Lay the gown on a surface where it would hold the whole gown with nothing drooping over the sides.
What type of hem is in the gown?
Is it tiny like rolled hem?
Does it have like a 2 inch hem?

When making shorter to knee consider a few inches below knee more of a classic length and chiffon fabric very light weight think it would hang better.

Depending on the color I would use a chalk marker, they brush off easily, mark all the way around the mark would of course have extra length included for the hem.
Use sharp scissors, cut along chalk line.

Will check in later to see what type of hem you have.

Hoping this is helping and I am wording it so you understand.

You are very wise to care enough to take precautions before diving right in. It is like painting a room more prep work than actual work.