In other place, I received this answer:
Export them before you close your twitter. https://help.twitter.com/en/managing-your-account/how-to-download-your-twitter-archive
This means you don’t lose anything, it’s just no longer public for other people to read.
If having those tweets is enough, you’re done, have fun on mastodon.
Otherwise try https://github.com/FGRibreau/import-tweets-to-mastodon
You’ll need git and npm installed, and then just copy those commands.
That tweets.js comes from your twitter archive. You can get the Mastodon API key from the settings on the mastodon instance you’re using (Settings>Developer>New Application).
Don’t wait for an easy graphical tool, you’ll be waiting for years.
The script author seems to be here:
Hey everyone, hope you are doing fine. I’m new and a noob about decentralized social networks like the Fediverse. So, sorry If this was asked before (and for my eventual mistakes, english isn’t my main language). I’ll give you some context: