Set number_of_extended_variable_length_records and info of spatial references

[lorusso86]

Hi, i have created a file Las from X,Y,Z points and it work. Now i wont set the info of spatial reference (latitude, longitude etc..). I have seen from specific of las files and i read that is important set the "number_of_extended_variable_length_records". I dont understand how set all other information (latitude, longitude etc..). can someone help me to set this information?? also an example is good. Thx