How to set a user to access on a db:

  1. Open Management Studio
  2. Go on properties of the server ex: SRV-2008\SQLEXPRESS
  3. Select mixed mode (SQL Server and Windows Authentication). Click OK.
  4. Always in Studio Management, add a new connection ( unroll security)
  5. In general, enter a name for the account and click on "SQL Server authentication". Enter a password and uncheck "Keep the password policy".
  6. Assign roles.

You can now use this user in a connection string or in credentials to remote a instance SQL.