Firebase rules: all users can read, authenticated users can create if they provide a uid, owner can delete, owner can update.

firebase-owner-rtdb-crud-rules.json

{
  "rules": {
    ".read": "true",
    ".write": "auth != null",
    "<ITEMS>": {
      ".indexOn": "<PROPERTY TO INDEX BY>",
      "$comment": {
        ".write": "(!data.exists() && newData.child('uid').val() == auth.uid) || (data.exists() && data.child('uid').val() == auth.uid && !newData.exists()) || (data.exists() && data.child('uid').val() == auth.uid && newData.child('uid').val() == auth.uid)"
      }
    }
  }
}

Authenticated users can write new data:
(!data.exists() && newData.child('uid').val() == auth.uid)
No existing data, new data being written has a uid property that matches the authenticated user's uid

Authenticated users can delete their own data:
(data.exists() && data.child('uid').val() == auth.uid && !newData.exists())
Data exists, the data has a property uid matching the authenticated user's uid, and no new data is being written

Authenticated users can update their own data:
(data.exists() && data.child('uid').val() == auth.uid && newData.child('uid').val() == auth.uid)
Data exists, the data has a property uid matching the authenticated user's uid, and new data being written also has matching uid

Notes:

Firebase RTDB security rules cascade, meaning additional access can be granted down the cascade, but access cannot be revoked. Most permissive rule must be at the top.