How to add toast style popup to my application?

This is pretty simple. You just need to set window in off-screen area and animate it's position until it is fully visible. Here is a sample code:

public partial class Form1 : Form
{
    private Timer timer;
    private int startPosX;
    private int startPosY;

    public Form1()
    {
        InitializeComponent();
        // We want our window to be the top most
        TopMost = true;
        // Pop doesn't need to be shown in task bar
        ShowInTaskbar = false;
        // Create and run timer for animation
        timer = new Timer();
        timer.Interval = 50;
        timer.Tick += timer_Tick;
    }

    protected override void OnLoad(EventArgs e)
    {
        // Move window out of screen
        startPosX = Screen.PrimaryScreen.WorkingArea.Width - Width;
        startPosY = Screen.PrimaryScreen.WorkingArea.Height;
        SetDesktopLocation(startPosX, startPosY);
        base.OnLoad(e);
        // Begin animation
        timer.Start();
    }

    void timer_Tick(object sender, EventArgs e)
    {
        //Lift window by 5 pixels
        startPosY -= 5; 
        //If window is fully visible stop the timer
        if (startPosY < Screen.PrimaryScreen.WorkingArea.Height - Height)
            timer.Stop();
        else
           SetDesktopLocation(startPosX, startPosY);
    }
}

For Customizable and Better looking notifications..

Check this link..

• Windows Toast Notifications - GitHub

There's support for notification balloons in Win32 (I'm not a .net programmer), with some useful properties as old new thing explains.

There's also a system wide semaphor which you should lock to prevent more than one popup from any application appearing at once.

There's a a couple of pages on the toast semaphor on msdn - the toast semaphor and in the broader context of usability. I also came across some example code to use the balloon api from C# while looking, but can't vouch for it.