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

Steam Trading Cards are collectable cards users get by playing games on Steam. The amount of trading cards must be between 5 and 15 cards, based on a theme for the game.

You need this to get it done:

Large Card Images
These act as "zoomed in" versions of the smaller card image.
File format: .jpg
Dimensions: 1920 x 1080 pixels
Must be less than 350KB
Small Card Images
These act as a view into the large image and will have the card title and frame overlaid onto it automatically.
File format: .png
Dimensions: 206 x 184 pixels
Game Logo
File format: .png (with transparency)
Dimensions: 206 x 44 pixels
Must be legible on both dark and light backgrounds.
Card Titles
These will be overlaid on the card.
Card Descriptions
A few lines of description or related game fiction that is shown with the card.
Border Colour
Entered as a hex code (ex: #FFFFFF)
Badge Images (x6)
When a user collects an entire set of cards, they are able to craft a badge (up to 5 times, not including the foil badge.) The badges should increase accordingly in desirability and value.
Each badge (Levels 1 - 5 and foil) requires an 80 x 80 pixel transparent .png
Badge Titles (x6)
Each badge requires a unique title.
Emoticon Images (x5)
Crafting a badge also grants a single custom emoticon which can be used in chat on Steam, or traded on the Steam Marketplace.
Each emoticon requires 2 transparent .png images: 18 x 18 pixels and 54 x 54 pixels. The 18x18 version is the most commonly viewed so ensure that the emoticon is legible at this size (emoticons are typically seen on #1B1B1B backgrounds.)
Emoticon Text (x5)
Each emoticon requires a title and unique entry text (e.g. :emoticon:)
Emoticon entry text can only use of alphanumeric (a-z, A-Z, 0-9) characters and the underscore ('_') character. No other characters are permitted.
Profile Background Images (x5)
Crafting a badge also grants 1 profile background for that game.
Profile backgrounds must be 1920 pixels wide with variable height, .jpg format and with a file size of less than 350KB.
Valve highly recommends transitioning at least the sides and bottom of the image to black so there are no visible edges to the backgrounds. See official Steamworks documentation for more rules/guidelines to follow.
Profile Background Text (x5)
Each profile background requires a title and description (a few lines of description or related game fiction that is shown with the background.)
Requirements missing or found errors? Report

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

Steamworks Documentation - Steam Trading Cards
Information about Steam Trading Cards and the graphical assets which are required for cards, badges, emoticons and profile backgrounds.
Link missing here or found errors? Improve How-Tos