Generally you should already know the scope of work, (part of being a general) what it entails, and the subs you are using i.e. have used them before and know their rates and production. Otherwise it can be a little bit of a guessing game in which case you would let the sub know what the allotted time frame and budget is, and see if they can meet those requirements. It’s not easy since every job is different and you will always run into a few hurdles along the way.