If you want to sticky a post, you need moderator privileges on the guild site in question.

When creating a new post, there will be an option to make it a sticky or an announcement right above the Submit button. The process looks about like this:

1. Click “New Post”
2. Look above the Submit button before you submit the post
3. Check the “Sticky” radio button
4. Submit the post
5. Done!

If the post already exists, then:

1. Edit the post
2. Look above the Submit button before you submit the post
3. Check the “Sticky” radio button
4. Submit the post
5. Done.