I find that achieving the best results when measuring can be overwhelming for some people.  I use to be one of those people.  Some people like to measure in their birthday suit and I say NAY.  Of course if you don't plan on wearing undies or a bra under your selected garment than by all means measure away in your birthday suit.  But, if you are like the majority you will want to measure in whatever selected undergarments that you would usually wear.  You want to hold the measuring tape so that it is comfortable and not sagging, but you don't want to hold it so tight that the finished garment will squeeze the life out of you either.  

Back Waist Length is measured from the first bone you can feel at the base of neck down to your natural waistline (see measuring waist).  Bust is measured  around the fullest part, right across the nipples and around your back.   Chest area is found directly under the arms (arm pitts) and around the back and completes above the bust.  Height is measured when you stand against a flat wall without your shoes, socks are optional.  All of you should be flat against the wall.  Don't stand on your toes for the illusion of more height, it will make your garment less attractive while you are wearing it.  Put a ruler on top of your head flat side on your head and ease out from below it.  You can hang a measuring take on the wall next to where you are standing, then you only need to look at it in relation to the ruler.  Hips are measured from around the body at their fullest part, usually 6 1/2" - 8 3/4" below your natural waist line. Waist is really simple use a narrow belt at your waist line and place it on the 1st notch where it is comfortable naturally.  Measure over the belt.

Once you have figured out all of your measurments write them down and put them aside.  If you are on a diet you may want to check them at least once a month.  Everyone should check before the buy anything online to be sure things have not changed either for the good or for the bad.  In this way you are assured that there will never be a bad fit. 

Most sewing patterns are for bust lines with a B cup as they have found this to be an average size.  So remember if a bust is listed as a 36 bust for a ready made dress, blouse or jacket etc or for a pattern it is also made for a B cup and you may have to take it or let out.  If it is a ready made garment be sure that there is enough room in the fabric for letting out.  Taking a garment in is usually the easiest.