Would like to add one more thing - Many times, organizations operate in decentralized models and so, there will be multiple resource managers whom you would need to approach to get their resources allocated to the project activities. They will be shocked if you go last minute to them and ask for resources.. So , the key is to start planning as soon as possible.

Also, before preparing schedule, first think of all holidays , time & logistics constraints across all geographical locations where your resources are located. This seems simple but if not addressed, can create very complex scheduling problems.

Thanks,
Rohit