Connecting SQL Server Developer Edition to Visual Studio Windows Forms Application
1) Create an application user for your app on the SQL Server instance.  Never use an administrative user or SA.

2) Create a DSN (Data source Name) on the machine that the app will connect to the SQL Server database.  This applies even if the two are on the same machine (which would be poor architecture, but possible in a solitary dev environment).

2a) The login credentials for this DSN should be the app's user in SQL Server

3) Go to ConnectionStrings.com and generate a SQL connection string based on the DSN you just made. (https://www.connectionstrings.com/dsn/)

4) Use the connection string in your app/web config and reference it in the SQLConnection call.