Taking your own measurements is very difficult. It's best to get help in order to be as accurate as possible. When taking measurements wear as little as possible. Underwear or a close fitting body suit works well. If that is impossible, make sure you love the fit of the pants you are wearing and make a note that you were wearing pants when you took the measurements.
Measurements for Custom Jeans
Waist to Hip
Waist to Seat
Waist to Floor
Measure at your natural waistline. Up where your back bone starts to bend. This will likely be higher than you wear your pants. Don't be surprised if the number doesn't match the size you're used to.
Measure the circumference at the widest part of your hips.
Measure the distance from your natural waist to the widest part of your hip.
Sit in a chair. Measure the distance from your natural waist to the chair.
Measure the distance from your natural waist to the floor on the outside of your leg.
Measure from the place where your pants' seams meet along the inside of your leg to the floor.