Some third party Mastodon apps have a built-in draft posts feature.
There is no official drafts feature in the Mastodon website interface, but unofficially you can use the Delete & re-draft option combined with DMs to create drafts:
- Log on through your server’s website and create your draft post, but don’t publish yet.
- Set visibility to mentioned people only, and don’t mention anyone.
- When you’re ready to save it, publish it.
- When you want to edit it, go to your Direct Messages tab and find the draft.
- Click the draft open, click ⋯ and select Delete & re-draft, the draft will open in the message editing window
- When you’re ready to publish for real, set the visibility to the correct setting and add any mentions you want, then press the Publish button.
…but, as many people have pointed out, it might be easier to just use a notepad app and copy and paste drafts from a text file on your computer or phone 😁