How this works: When you login with both your Twitter account and an account on a Mastodon instance Twitodon will remember that your Twitter account is linked to the Mastodon account. When someone else comes along and does the same, if they follow you on Twitter then their CSV file will include your Mastodon account for them to follow because you have created the association. This does inevitably mean that if someone you follow on Twitter has not signed in to Twitodon with both their Twitter and their Mastodon accounts to create their association in Twitodon's database then Twitodon doesn't know about the association to offer you their Mastodon account via your CSV file download.
Step 1. Login With Twitter.
Step 2. Login With Mastodon.
Enter your Mastodon host's web address:
Looking for Mastodon users progress, scanned 0 of 0 users you follow on Twitter. Discovered 0 Twitter users already on Mastodon.
Step 3. Download matching users in CSV format to import into your Mastodon account. Click Here to download the CSV file
Navigate to your Mastodon instance's import page: https://mastodon.social/settings/import
new_mastodon_follows.csv, that you downloaded from Twitodon.