Taking careful measurements is important to get a great fit on your soakers and longies. Measurements should be taken over the cloth diaper that will be worn under the soaker. 

Rise is measured from just above the back of the diaper, through the crotch, and up to the front above the diaper.  Allow plenty of room in the rise (I like to move the baby's legs around and especially pull them up when measuring as this can often pull the soaker down in the back)

Hip is measured at the widest point across the hips. It's helpful to take several measurements and use the largest.

Inseam is easiest measured by measuring the inside leg from crotch to ankle. It is often easier to measure the inseam of a pair of pants that fit the way that you would like, rather than measuring a squirming baby.

Pants:

Soakers: