You applying for LE Commision? I assume the criteria will differ depending on who you want to commision with. And if you are asking for a copy of a letter of introduction i suggest you brush up on your defence writing at the very least.
Get in touch with your local Army Education Centre. They should be able to give you relevant advice and an up to date DIN with all criteria that you need to satisfy to apply for a commission. I'm in the process of producing an LE pack at the moment: most LE commissioning courses follow a pretty similar(ish) format based on RCB so the pack should be generic.