Generate a gpg key if you don't have one. Generate a password in your password manager to use and store the private key in your pw mgr when you're done:
gpg --full-gen-key

Copy the fingerprint of your generated key:
gpg --fingerprint

Generate an initial public key to export with an email you can verify:
gpg --armor --export EMAIL > pubkey.asc

Upload pubkey.asc to keys.openpgp.org:
https://keys.openpgp.org/upload

Confirm your verification email after it uploads.

Add a tweet on birdsite, include the text, replace with your fingerprint with no spaces:
[Verifying my cryptographic key: openpgp4fpr:FINGERPRINT]

Copy the link to your tweet for later use.
https://twitter.com/USER/status/TWEET_ID

Add the same verification string to your infosec.exchange profile.

Copy the link to your Mastodon profile for later use.
https://infosec.exchange/@USER

Now we go back to your gpg to add notations to your key.
gpg --edit-key FINGERPRINT
uid 1 <enter>

We'll be adding two notations:
notation <enter>
[email protected]=https://twitter.com/USER/status/TWEET_ID
notation <enter>
[email protected]=https://infosec.exchange/@USER
save <enter>

Now we can upload the new key easily:
gpg --keyserver hkps://keys.openpgp.org --send-keys FINGERPRINT

Once that's done you can validate at your keyoxide fingerprint page:
https://keyoxide.org/FINGERPRINT

You should see check marks for Mastodon and birdsite validating ownership.

If anyone is unsure whether you own the account asking for proof you can direct them to your keyoxide page.

If I missed steps let me know but you can also read the docs for more features and service providers: https://docs.keyoxide.org/