In general, most book bloggers send out requests after they've hit the 6-month mark. Before is fine, but make sure that you have enough posts and followers to make a publisher go "Oh, I'd want to approve that blogger for (book title)."
Netgalley and Edelweiss
In Netgalley, requests are sent out when you click "request". An email is sent to the publisher saying that you've requested a book. It's at the publisher's discretion whether or not to approve of your request or deny it. Therefore, it's important that all the necessary stats are included in the request.
When a request is sent out in Netgalley, the publishers see your profile and the short biography that you typed up (in the profile section). As a general rule of thumb, all requests (whether through email, Netgalley or Edelweiss) should have the following information:
-Who You Are
-Name of Blog/Publication Outlet
-Stats (Monthly pageviews, Twitter followers, etc.)
-Links to blog, Twitter, and other outlets that have to do with your blog
-Title of Book You're Requesting (Include Author, Pub Date, etc.)
Edelweiss is pretty much the same thing, however, users have to write a request for every book that they request. It's a little bit more of a hassle, but it allows for a personalized request.
Personally, I like to personalize my requests as much as I can. I can't do so in Netgalley but when I send out email requests, I like to write the reasons why I'm interested in the book that I'm requesting. Nothing makes me more annoyed than receiving a copy and paste email and I can imagine that publishers are the same.
When you want to request a physical copy of the book, you can do so by emailing the publicity department of the publisher of the book. The email can be found on the websites of the publishers.
For email requests, it's important to know that publishers only have a limited amount of books and that not every request will be approved. Don't keep emailing the publisher back if you haven't heard from them and don't request too many books from the same publisher in one publishing season.
As I said earlier, I like to personalize my email requests. I like to state the reasons why I'm requesting (what I find interesting about the book etc.) It only takes a little bit more time but it's worth it.
Important note: Any type of request is a professional contact with the publisher. What you write and how you write the request is a mirror of who you are and what your blog is. Keep it short and professional.
If you're sending a request via email, email it 3-6 months before the publication date of the book.
That's it! I hope this clears up some questions you may have and if you still have any questions, feel free to comment below and I'll respond!