This is an updated version of jgf's instructions, which appear to be out of date in that any users can now create contest instances.

Terminology
- A contest instance is a collection containing the questions from one year of a particular competition.
- A contest is a collection containing all of the contest instances related to a particular competition.
For example, IMO is a contest containing contest instances, such as 2015 IMO and 2014 IMO.

1 - Post contest problems
For each problem, create a separate new topic in the High School Olympiads forum. The first post should only contain the problem statement (source is optional). This is an example.

2 - Get problem links
For the next step, you will need to obtain direct links to each of the topics you just posted.
The links will look something like for example: Note that you want the URL of the first post, rather than the topic (you can tell this is correct of the letter "p" in the URL). To get the direct link to the 1st post, click on the #1 in the top right corner of the 1st post. That will bring up a modal from which you can copy the direct link.

3 - Create contest instance
Now that you have links to the problems, you can create the collection by going to your My Collections page and clicking on the + in the top right. In the modal that pops up, you want to select the Posts option.

This will bring up the Create Collection page. For the Collection Name, the format should be

<Year> <Contest>

The Contest name should match the name in the Contest Collections. For example, "IMO" instead of "International Mathematics Olympiad."

You shouldn't need to change any of the other settings, but here's how they should be set:
- Public/Private: Public (we want any user to be able to read this collection)
- Banned: None (no one is banned)
- Status: Locked (we want only collection administrators and moderators to be able to edit the contents of this collection)

Click CREATE in the bottom right. This will bring up the Collection Info page. (If you ever need to get back to this page, for example to edit the collection, click the Information icon 3 next to the collection name.)

Click EDIT CONTENTS in the bottom right.

For each problem, add it to the collection by doing the following:
- Set Select Post Type to Forum Post.
- For Post Label, put the problem number.
- For Forum Post, paste the direct link to the 1st post from before.
- Click ADD TO COLLECTION.

You may also add custom text, for example: "Day 1," "Day 2," "Final Round," "Morning session," "Number theory," etc.
- Set Select Post Type to Custom Text.
- Post Label is usually left blank. (This is label that is shown in the left box with the problem numbers.)
- For Custom Text, put whatever makes sense.
- Click ADD TO COLLECTION.

You can rearrange the order in the contest instance by dragging-and-dropping the components. After you're done, click SAVE.

4 - Important: Set privileges
You also want to set the permissions to your contest instance. Go to the Collection Info page for your contest instance. Under Permissions, in Inherit Roles From, start typing the name of the contest (the same contest you just added the contest instance to). It should autocomplete so you can select the contest from the list.

5 - Send moderator a request
Now you are all done! Once you have created a contest instance, all that remains is to have a moderator add it to the appropriate collection.

For contests that I moderate (like USA and Taiwan, among others), feel free to send me directly a link to your contest instance, and I will add it to the appropriate place. For other contests, if you post a link at the end of this thread then some other moderator will eventually handle it (but this is in my experience slower).