What is the method to select a contractor?

according type of the project that this project requires some contractors in determined field , you should to make selective tender between nominated contractors , if else make it as open tender . in addition to make arrangements to negotiate and exchange offers & proposals  

1- look at his portfolio & historical experiences .  

2- their situation in running Project . 

3- have a look to his break down prices if it rational . 

4- ask them to proof his manpower quantities .