This is easy to do, but nearly always a bad idea. I've seen it done twice, and in both cases it caused an audit failure which cost the DBA their job within two years.

1) Backup the model database
2) Create a SQL Server login.
3) Grant that login access to every database (including model)
4) Make the users created in step 3 members of the database's db_datareader role.

-PatP