I would talk to whomever you are planning to buy the tank from. Each kiltmaker uses a slightly different system and are usually quite patient at explaining what they need.

As a general guideline you normally measure around your belly button with a flat hand between you and the tape. You also need the widest part around the bum and finally a measurement from your bellybutton to mid-knee. The best way to do this is get someone to have you kneel and measure from your belly button to the floor. NB don't look down. that shortens the measurement but an inch or more.

Hope that helps