In applying the high-low method of cost estimation to mixed costs, how is the total fixed cost estimated?

Mixed cost also called the semi variable cost . Mixed cost contains both fixed and variable cost .

Example - Electricity charges ( Minimum fixed charge plus consumption charge)

Fixed cost - Fixed cost is the cost that remains constant irrespective of the production activity of the entity . 

Example - Office rent , Salary to Office staff , Watch man salary etc

Variable cost - Variable cost changes with the change in number of units of production.

Example - Direct Material , Direct labour etc