Action Details
Requirements and how-to articles for Steam's action:

Early Access enables you to sell your game on Steam while it is still being developed, and provide context to customers that a product should be considered "unfinished." Early Access is a place for games that are in a playable alpha or beta state, are worth the current value of the playable build, and that you plan to continue to develop for release.

You need this to get it done:

Early Access Q&A
When you indicate in Steamworks that you wish for your product to be listed in the Early Access section of Steam, you must answer the following questions:
Why Early Access?
Approximately how long will this game be in Early Access?
How is the full version planned to differ from the Early Access version?
What is the current state of the Early Access version?
Will the game be priced differently during and after Early Access?
How are you planning on involving the Community in your development process?
(Your product page and Q&A will be reviewed by Valve once they are completed.)
Requirements missing or found errors? Report

How-To Articles
A list of useful links for you:

Steamworks Documentation - Early Access
Information about Steam's Early Access tool.
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