How should I go about commissioning in another branch of the military or USMC as a marine officer?

Do a little homework and create a checklist of the necessary forms and when they can be done and by whom. Expect to do all the paperwork yourself. Inform your chain of command that you are interested in pursuing this career avenue. Seek out leadership opportunities. Draft a letter of intent to present to your senior leadership. The process could take about a year, so backwards plan from the time you will need to transition (late summer).

With regards to the packet, I'm not sure what the USMC needs specifically, but I can assume they will need the following: Get an official photo in your Class As from the local official photographer. Take the Secure copies of your APFT card and any award certificates. You'll need to get the appropriate medical exams/physicals (probably a phase I/II pre-commissioning exam). Get letters of recommendation from senior NCOs and officers in your CoC. Take the appropriate college entrance examinations, such as the COMPASS test, SAT, ACT, and a copy of your ASVAB scores to be sure. Secure a copy of your college admissions document once accepted. I recommend also getting letters of recommendation from the gaining USMC ROTC unit and from a professor at the school you intend to study from as well. Print off a copy of your Enlisted Records Brief (Make sure it gets endorsed by your S-1). Also, create a supporting document (there is a DA form for this) detailing how you will pay for school and living expenses while you are a Cadet. You will need to provide your background check information as well (those documents can be found in your accessions packet from Basic Training)

MAKE MULTIPLE COPIES OF EVERYTHING. Have a second and third copy of you packet standing by if the first one gets lost and for your own records. Once you complete your packet, FOLLOW-UP with your chain of command and their administrative staff to track your packet as it moves from the offices of your Platoon to Company to Battalion to Brigade. Use a tracking document so that when you turn in your packet the receiving personnel must sign for it.

And when I was USAF OTS after initially Army ROTC, many guys were intersvc while I was at Lackland, it's at Maxwell now, if you hadn't known, it was moved some time ago...one guy who'd been Navy enlisted I'd chatted with even had Navy enlisted dolphins on his USAF blues, the only time I'd ever seen that....

http://www.ocsfoundation.org

You'll find this useful, I think....