Before you're able to post a listing, are you already an administrator of your Idealist.org profile page? If not, check out Becoming an Administrator of your Organization's Idealist Profile. Once you're an administrator, just log in and click "Post a Listing" at the upper right of any page and you should be taken to your organization's dashboard.
From here, click on the blue "Post a Listing" button and select the type of listing you're looking to post; you'll be able to fill out a short form with information about your opportunity. Be sure to fill in all of the form fields with asterisks- those are the required ones! Once you've got the required fields full, you can Save a Draft (at which point you can come back at any time to finish), Add to Cart if this is a paid listing, or Publish using the buttons at the bottom.
If this is a paid listing, you'll first select the billing information from the list, or scroll to the bottom and select "Add a New Invoice Address" where you can fill in the correct information. You'll then be taken to your cart where you can either Pay by Credit Card or Pay by Check. If you have any trouble with this process, please contact us and let us know. We're happy to help!