Download the zip from the github releases and create a html document then use the library as you would with the normal canvas API.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>ctxsvg test app</title>
</head>
<body>
<svg height="400" width="400"></svg>
<script src="ctxsvg.min.js"></script>
<script>
let ctx = new ctxsvg.SVGContext('svg');
ctx.beginPath();
ctx.moveTo(0, 0);
ctx.lineTo(200, 100);
ctx.lineTo(300, 200);
ctx.lineTo(400, 400);
ctx.strokeStyle = 'green';
ctx.lineWidth = 5;
ctx.closePath();
ctx.stroke();
</script>
</body>
</html>