You go out and shop for a set of attic stairs. Then, when you find something you like, you buy it and read the instructions.
I don't think anyone really wants to get into the details of how to do this because there are load-bearing wall issues, installing it in a non-load bearing wall scenario, and so on. Depending on how your attic was built and where the loads are will dictate how to do this job.
If you're asking the type of question you're asking, it would be prudent to hire a professional. Maybe you know a friend that's a carpenter, or a friend of a friend, where you can buy beer and pizza, or somethin'. Just work it out with whoever. Your best 'deal' is to find someone willing to do it as a side-job.
Here's an old post that might help you in your search for stairs. Attic Stairs - My best to ya and hope this helps.