using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    class CustomToolTip : ToolTip
    {
        public CustomToolTip()
        {
            this.OwnerDraw = true;
            this.Popup += new PopupEventHandler(this.OnPopup);
            this.Draw += new DrawToolTipEventHandler(this.OnDraw);
        }

        private void OnPopup(object sender, PopupEventArgs e) // use this event to set the size of the tool tip
        {
            e.ToolTipSize = new Size(200, 100);
        }

        private void OnDraw(object sender, DrawToolTipEventArgs e) // use this event to customise the tool tip
        {
            Graphics g = e.Graphics;

            LinearGradientBrush b = new LinearGradientBrush(e.Bounds,
                Color.GreenYellow, Color.MintCream, 45f);

            g.FillRectangle(b, e.Bounds);

            g.DrawRectangle(new Pen(Brushes.Red, 1), new Rectangle(e.Bounds.X, e.Bounds.Y,
                e.Bounds.Width - 1, e.Bounds.Height - 1));

            g.DrawString(e.ToolTipText, new Font(e.Font, FontStyle.Bold), Brushes.Silver,
                new PointF(e.Bounds.X + 6, e.Bounds.Y + 6)); // shadow layer
            g.DrawString(e.ToolTipText, new Font(e.Font, FontStyle.Bold), Brushes.Black,
                new PointF(e.Bounds.X + 5, e.Bounds.Y + 5)); // top layer

            b.Dispose();
        }
    }
}

using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        private CustomToolTip tip;

        public Form1()
        {
            InitializeComponent();
            
            this.tip = new CustomToolTip();
        }

        private void button1_MouseHover(object sender, EventArgs e)
        {
            this.tip.SetToolTip(this.button1, "this is a test");
        }
    }
}