Hi viktoria,
you have to write your own Authentication Provider.
You start with a blank module. You add a UserControl named Login.ascx.
The class in the code behind has to inherit from AuthenticationLoginBase.
You need a login button which starts the login process.
Have a look at DesktopModules/AuthenticationServices/DNN
Don't forget that you have to add a licence.txt file to your package. This is required due to the authentication provider installation
process.

Finished development you package the module and install it as a additional Authentication system. Running DNN 4.9.* you have to put the .dll into the Providers subfolder.

Hope that helps,
Matze