You can pin posts to your Mastodon profile if you are using Mastodon's web interface, Mastodon's web app or a third party Mastodon app:

1. Go to one of your posts which you want to pin
2. Click ⋯ on that post
3. Select "Pin"

You can unpin a post by repeating this but selecting "Unpin".

Lots more info about pinning posts on Mastodon at fedi.tips/how-do-i-pin-my-post

The lack of a "Pin" option on the official apps is yet another reason to switch to an alternative: fedi.tips/which-apps-can-i-use

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If you have a fediverse account, you can quote this note from your own instance. Search https://social.growyourown.services/users/FediTips/statuses/115702125225264385 on your instance and quote it. (Note that quoting is not supported in Mastodon.)