Action Details
Requirements and how-to articles for itch.io's action:

Hosting a game jam is a great way to bring together game developers for an event or cause, and itch.io allows users to create and run game jams for free.

Requirements
You need this to get it done:

Theme
Most game jams are based around a theme which is revealed when the jam begins. Entrants try and make use of this theme the best they can, basing their game around it.
Contributors & Hosts
itch.io has a contributors system which allows you to give other users access to your jam, with each type of contributor having different permissions:
Host — Shown on the jam’s page as being a host
Admin — Allowed to edit the jam and submissions just like the original creator
Judge — Allowed to vote on entries, even if they didn’t submit. You can combine this with judge only voting to limit voting to a specific set of people.
You can also elect moderators for your jam's community page.
Ranking Criteria
When creating a jam you have the option to pick the criteria that entries are rated on (e.g. gameplay, use of theme, etc.) You can add as many as you like and they can be whatever you want. Each criteria is rated on with a 5 star system. By default, entries are ranked by the average score of all the criteria. If you'd like to have one of your custom voting criteria determine the overall ranking then you can select it as the Primary option from the criteria editor.
Custom Fields
In addition to criteria, you can also provide custom fields that are required along with a submission. For example if source code a requirement for submission you can create a "source code" field, or if your game must be released under Creative Commons you can create a drop-down for that. Just like criteria, the custom fields are free-form so you're free to make them whatever you want.
Voting Permission
In a ranked jam you can control who has permission to vote:
Restrict to only those who have entered into the jam (optionally including any contributors to games for team efforts)
Approved judge accounts
Anyone with an itch.io account
Submission Start Date & Time
The date and time when the jam begins. This is when the theme is revealed and submissions open.
Submission End Date & Time
The date and time when submissions close. (itch.io allows you to generate late-submit links for any submissions which missed the deadline.)
Voting Start Date & Time
In a ranked jam, this is the date and time when voting begins. This generally begins straight away after the submissions close.
Voting End Date & Time
In a ranked jam, this is the date and time when voting ends and results are calculated and announced.
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How-To Articles
A list of useful links for you:

0
itch.io Documentation - Hosting a Game Jam
Information about hosting a game jam on itch.io.
Official
Link missing here or found errors? Improve How-Tos