how to do crossover and mutation to produce new offspring (in coding) , on what basis we choose crossover point does one have to do it manually ,

I am trying to minimize a function using genetic algorithm , the fitness function here does not contain variables in direct but the variables of fitness function are generated from another function using other variables (the variables that i need )

1st set of variables(3 different values) ---> generates another set of variables(2nd) ---> fitness is calculated using 2nd set of variables .

I have managed to input 1st set of population (of 1st variables)and calculated their fitnesses and stored them in an matrix,

now i want to generate new offsprings from 1st population by crossover and mutation with the help of fitness values ,

I am using binary encoding of variables , and now i want to know how to crossover and mutate the values and produce the new offspring ,

Hence my population of 1st set of variables are in binary and i want to produce the new offspring