How are military missions set up?

The PLCOMD gets a fifty page brief which you have to sit through for an hour. That's your orders. The PLCOMD instructs the PLSGT on what's gonna happen, and the PLSGT instructs the SECCOMDs. Then you're a given a time to parade for inspection before deployment.

The PLCOMD develops the battle plan and coordinates his forces. The PLSGT does this in his absence. A lieutenant commands the platoon, a sergeant is his/her 2IC. The platoon is made of four sections (or squads in yank money) of eight blokes, and each section has two fireteams of four blokes. Each fireteam has a lance jack to lead it, and each section has a full screw (corporal) to lead it.

For the Air Force (or any other branch with air assets), you're given what's called an ATO (Air Tasking Order). Basically, it's a list of objectives for the next 24 hours.

The overall mission of each branch's air asset is decided probably by the theater commander (probably a 3 or 4 star general/admiral). Once that has been decided, individual missions or sorties are planned out and written on this document.

From Wiki:

A method used to task and disseminate to components, subordinate units, and command and control agencies projected sorties, capabilities and/or forces to targets and specific missions. Normally provides specific instructions to include call signs, targets, controlling agencies, etc., as well as general instructions.

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The pilots are briefed on the mission. What your objective is, what main objective for the entire sortie is, what other component's objectives are, when you are going to assemble en route if you're doing so, threats to you or other assets either from the ground or air, who is going to be on station, where the closest aerial tanker is going to, what radio frequencies to be on, your call sign, friendly forces or enemy forces in the area, what altitude to fly at to avoid threats, what time you take off, what time you need to be on target, etc.

PS...it's not about rank. It's about position. Just because you're a general doesn't mean you do that stuff. Sometimes also, a subordinate such as a colonel may be tasked to come up with a plan and submits it to the theater commander (in this case a 4 star general) for approval.

At what level numbnuts?

The planning process for a Platoon of 30 troops is significantly different than for a Division of 18,000 troops which is significantly different than for a Coalition Force made up of 20 nations and 500,000+ men

Very delicately.