you can get a basic apron pattern for fabric and work off that to customize it for leather. The pattern wont know and wont care. Or you can get a basic apron from a thrift store and deconstruct it.
Or start at your belly button measure down how long you want it, measure up how high you want it, add the two cut a big rectangle out then measure nip to nip or there about and how far under your arm you want it, or down to your hip where the tie will be. use these to cut out the upper parts . then add at your discretion the pockets or what ever. i would use some paper to make the first pattern then transfer to leather after you get the dimension correct.