Ideally ask for help taking measurements, if doing on your own stand in front of a mirror to ensure tape measure sits even. Measure with non-padded bra, thin layer of non-control garments to ensure a good fit, be honest with measurements.
A - bust circumference, please also include bra and cup size.
B - under bust circumference. If your bust is heavy run the tape measure under it, this measurement should be taken 1 inch down from crease under bust where bust joins torso.
C - waist circumference, feel below lowest rib and above hip bone for this point, also let me know if this area is squishy or firm on you.
D - high hip circumference
E - low hip circumference, this is usually the fullest part.
The length measurements sit between the circumference measurements, to get this exact it can be helpful to mark with a pin where you laid the tape for the circumference measurements.
F - bust point to top edge of corset, your bust point is where you want your nipple to sit, if you want some lift hold your breast up to where you want it to be and have the bust point in the middle of your breast, keep holding it to take measurement G.
H - under bust to waist, please indicate if planning an under bust corset and you want the corset to sit a little lower at this point so it doesn't push your breast up too much when sitting.
I - waist to high hip, this should also be the same as waist to where your leg/thigh comes when you are sitting, once again so the corset doesn't push up when sitting
Photos and even a Skype are really helpful also, so I can see where your curves sit. As with what you are wearing to take your measurements for these you should wear a thin layer of form fitting but not compression or overly tight clothing.