https://github.com/dipongkor/FBA-Custom-Login-Page

Step 1. From Sharepoint Central Administration page. Click New to create a New Web App.

 - Creat New Web App continue,

Step 2. Start “aspnet_regsql.exe” to connect SQL Database and follow screenshots.

Step 3. From IIS, configure “Connection and Providers”.

** Follow the same Connection and Providers to "SecurityTokenServiceApplication"

Step 4. Back to Sharepoint Central Administration page, then enable “Forms Based Authentication” for created new application on step 1.

Provider names are created providers on IIS in step 3.

Step 5. Go to Sign In page to check “Authentication Page” shows as expected.

Step 6. In IIS setup, Set Default Provider for Roles and Users.

Step 7. Add preferred users to be “Forms Authenticated”.

Step 8. Back to login page and test login.