//1)Style tag goes in header     
<style>
        canvas{ border: 1px solid black; }
 
        </style>

//2)Add following to body tag

<canvas id="myCanvas"  ></canvas>

//3)add following to top of Script tag

   var canvas = document.getElementById("myCanvas");
    var ctx = canvas.getContext("2d");
        ctx.canvas.width  = 500;
        ctx.canvas.height = 250;
 
 //4) add following to script tag
 function draw() {
            ctx.clearRect(0, 0, canvas.width, canvas.height);
             ctx.fillStyle = "gray";
            ctx.fillRect(0, 0, canvas.width, canvas.height);
 
}

//5 controls speed of animation - smaller number the faster
setInterval(draw, 10);


//6) draw Circle to screen
        ctx.beginPath();
        ctx.arc(100, 100, 40, 0, Math.PI*2, false);//40 is the size
        ctx.fillStyle = "green";
        ctx.fill();
        ctx.closePath();

//END