I give both the Non Stretched measurement and Stretched measurement of the jeans in my listings. The Non Stretched measurement will be most like a pair of your own jeans laid out flat and measured; The Stretched measurement will be most like measuring around your own body.
Inside Leg - Measure from your crotch down to your ankle (or wherever you want your jeans to fall), remembering you may want to measure a little longer if you want to wear heels.
Waist - There may be a difference between your actual waistline, and where your jeans will sit. As fashion changes, your jeans may sit on your hips, waist, or below your bust, and you will need to measure where your jeans will sit, not necessarily your natural waistline, which will be a few inches above your stomach button (between the top of the hip bones and lower ribs).
The rise of the jeans will determine where your jeans will sit, so if for example your jeans have a 8 inch rise, you will need to measure 8 inches up from your crotch, and then from this point measure your ‘waist’, measuring your body keeping your tape as straight around your body as possible.
Hips - Look sideways into a mirror, keep your legs together, and find the widest part of your bottom, and this level is your hip measurement. If measuring flat, around 1 inch from the crotch.
Thigh - Measure the circumference of the top part of your thigh.
So, if in doubt, measure from your own body.
I also state what UK Jean Size it is, so this should give you a good idea whether they will fit.
Good luck measuring! If you have any questions or suggestions about any aspect of Petite Legs, please don't hesitate to contact me! I will be more than happy to give you personal help and advice :)