Re How to measure. Options:
What size am I? Simple. Just lay your old favourite jeans down flat and measure straight across the back waistband. Write the measurement down. See photo.
2. Inside Leg
Measure from the bottom of the hem (the bottom of the leg) and up to the crutch. That will give you the inside leg measurement. See photo for help.
3. Front rise
It might help to look at the photos. But with your jeans flat, measure from overlapping seams in the crutch, to the top of the front waistband. Pretty simple.