Depends the purpose you have. The are a lot of kits for mtDNA extraction, but you also may use some simple steps:

1) DNA extraction from tissues containing mitochondria, but not nucleus. For example, hair (except hair bulb). In human CUTED hairs (except hair bulb) only mtDNA present, i had an experience in forensic mtDNA isolation from human hair shafts, but not sure the same will be with other animals.

2) mitochondria gradient extraction from tissues - simple method with easy reagents and equipment. After mitochondrial fraction isolation you may use all methods for total DNA isolation

3) The simplest (we used it for mamalian (catle) mtDNA NGS) - total DNA isolation then - agarose gel phoresis (0,8-1,2%) then - extraction mtDNA from gel.