I sew for myself, so I have a lot of practice making sure the I get the right measurements for myself, and I use this when taking measurements for weight loss.
For starters, it's always easiest if you are looking at yourself in the mirror while measuring.
Circumference measurement-
Guideline
Breast- nipples
Chest- right under boobs
Waist- narrowest part of torso under the last ribcage
(on some/most people, at elbow level)
Belly button- mmm.. belly button
(I measure these differently, because on me it is about 3 inches below my waist. It is usually below the waist)
Hip- hip bone
(this is a few inches below your belly button; you might have to feel for your hip bone)
Widest part of lower body- crotch
(Believe it or not, this is usually the widest part of the body, it will include saddlebags, hips, butt and maybe lower belly)
For legs, I always use the tape measure and measure down from the crotch, then measure around at various points such as upper thigh, mid-thigh, upper knee, calf. When measuring down from the crotch, always go straight down-
do not follow the lines of the body or as you shrink, the measurement will change.
The points down will obviously be different for everyone, because of length of limbs. Just as long as you're consistent with yourself.
For arms, I use the same concept of the legs but with a different guideline.
I usually use the mid elbow (crease) as a guide for both the mid-arm and the forearm.
Another suggestion that might help, if you have a dedicated mirror, you can use a dry erase marker or tape and mark the lines for the most difficult areas, such as mid thigh and use waist and crotch lines as guidelines. I wouldn't use the shoulder or head as a guideline because your posture might get better and through things off. Also, make sure you are standing the same distance away from the mirror each time.
Hopefully this helps