Design doc

I'm looking for a skilled designer to create an architectural design document for a new project. Although I don't have a specific style or layout in mind, I do have a general idea of what I'd like to accomplish. I need someone with experience in the area to bring my vision to life. I need a detailed sequence diagram for authenticating a user. Basically the user will enter his id and password in an app. I have a service. Need to write code to authenticate the user against the credentials that are stored in the db. This should happen in a secure way using a key.